Disable -Werror=implicit-fallthrough on GCC7 and later.  Taken from upstream:
https://github.com/Tencent/rapidjson/commit/fe19b7b6016d446722621fb407738209d1a911e8
https://github.com/Tencent/rapidjson/commit/cba45fe9de6923b858edb0780e257b7257aa4f7b

diff --git a/include/rapidjson/internal/regex.h b/include/rapidjson/internal/regex.h
index 422a5240..d4039716 100644
--- a/include/rapidjson/internal/regex.h
+++ b/include/rapidjson/internal/regex.h
@@ -29,6 +29,9 @@ RAPIDJSON_DIAG_OFF(implicit-fallthrough)
 #ifdef __GNUC__
 RAPIDJSON_DIAG_PUSH
 RAPIDJSON_DIAG_OFF(effc++)
+#if __GNUC__ >= 7
+RAPIDJSON_DIAG_OFF(implicit-fallthrough)
+#endif
 #endif
 
 #ifdef _MSC_VER
ect name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=e13b354a7733d22bb67c8618aa41d7f759f4432e'>refs</a><a class='active' href='/guix/log/tests/swh.scm'>log</a><a href='/guix/tree/tests/swh.scm?id=e13b354a7733d22bb67c8618aa41d7f759f4432e'>tree</a><a href='/guix/commit/tests/swh.scm?id=e13b354a7733d22bb67c8618aa41d7f759f4432e'>commit</a><a href='/guix/diff/tests/swh.scm?id=e13b354a7733d22bb67c8618aa41d7f759f4432e'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/swh.scm'>
<input type='hidden' name='id' value='e13b354a7733d22bb67c8618aa41d7f759f4432e'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=e13b354a7733d22bb67c8618aa41d7f759f4432e'>root</a>/<a href='/guix/log/tests?id=e13b354a7733d22bb67c8618aa41d7f759f4432e'>tests</a>/<a href='/guix/log/tests/swh.scm?id=e13b354a7733d22bb67c8618aa41d7f759f4432e'>swh.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/swh.scm?id=e13b354a7733d22bb67c8618aa41d7f759f4432e&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-09-02 15:25:01 +0200'>2019-09-02</span></td><td><a href='/guix/commit/tests/swh.scm?id=ba1c1853a79a5930ca7db7a6b368859f805df98d'>swh: Add hooks for rate limiting handling.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-02 15:25:01 +0200'>2019-09-02</span></td><td><a href='/guix/commit/tests/swh.scm?id=9323ab550f3bcb75fcaefbb20847595974702d5b'>tests: 'with-http-server' accepts multiple responses.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-22 11:53:39 +0200'>2019-07-22</span></td><td><a href='/guix/commit/tests/swh.scm?id=96f1cbeff84819f9886d15763b4c477cdecd7784'>swh: Add basic tests.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
-08-29 16:24:17 +0200'>2020-08-29</span></td><td><a href='/guix/commit/m4/guix.m4?id=f5d3a1690cb78ce9413b78352f1030bf4801617a'>build: Remove check for Guile 2.2.1 bug.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-08-24 13:35:24 +0200'>2020-08-24</span></td><td><a href='/guix/commit/m4/guix.m4?id=4c0c65acfade63ce0549115d19db4b639c1e9992'>Use "guile-zlib" and "guile-lzlib" instead of (guix config).</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-07-20 23:25:09 +0200'>2020-07-20</span></td><td><a href='/guix/commit/m4/guix.m4?id=e8088f0b06c2193f2cce04a48aa1350229442a9f'>ssh: Speed up RPCs by using #:nodelay.</a><span class='msg-avail'>...</span></td><td>Lars-Dominik Braun</td></tr>
<tr><td><span title='2020-05-28 11:12:40 +0300'>2020-05-28</span></td><td><a href='/guix/commit/m4/guix.m4?id=03e6404936ef0974fdbc877d537b97b28e54124d'>doc: Remove explicit support for mips64el-linux.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-05-28 00:55:00 +0200'>2020-05-28</span></td><td><a href='/guix/commit/m4/guix.m4?id=c1dc50ab202fafedde3a6b70ce6339496ad53a51'>maint: Check whether Guile-Gcrypt is recent enough.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-02-22 00:45:58 +0100'>2020-02-22</span></td><td><a href='/guix/commit/m4/guix.m4?id=378918725259d221222dd0b1831fbaa3e0a17e1f'>build: Depend on guile-ssh 0.12.0</a><span class='msg-avail'>...</span></td><td>Lars-Dominik Braun</td></tr>
<tr><td><span title='2019-08-17 19:26:21 +0200'>2019-08-17</span></td><td><a href='/guix/commit/m4/guix.m4?id=611a64bd7e0893057cd04dcdc2a8d5e7ecc39e45'>build: 'GUIX_CHECK_GUILE_JSON' really checks for Guile-JSON 3.x.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-25 00:16:41 +0200'>2019-07-25</span></td><td><a href='/guix/commit/m4/guix.m4?id=81c3dc32244a17241d74eea9fa265edfcb326f6d'>maint: Switch to Guile-JSON 3.x.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-05-06 23:21:33 +0200'>2019-05-06</span></td><td><a href='/guix/commit/m4/guix.m4?id=fea338c6ca1922097fa233be85f424c152a4f507'>Add (guix lzlib).</a><span class='msg-avail'>...</span></td><td>Pierre Neidhardt</td></tr>
<tr><td><span title='2018-11-23 15:42:00 +0100'>2018-11-23</span></td><td><a href='/guix/commit/m4/guix.m4?id=60e1c1099fc3d73ed7d3235e71aae5d00ab7d773'>Update Guile-SQLite3 URL everywhere.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-09-04 17:25:11 +0200'>2018-09-04</span></td><td><a href='/guix/commit/m4/guix.m4?id=ca719424455465fca4b872c371daf2a46de88b33'>Switch to Guile-Gcrypt.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-06-01 15:35:15 +0200'>2018-06-01</span></td><td><a href='/guix/commit/m4/guix.m4?id=d59e75f3b5b82692bd250a1a3a9965397bb588c5'>build: Check for Guile-SQLite3.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>