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
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<div class='path'>path: <a href='/guix/log/?id=a251c7890727031840b90f6f83c9016d75e99dfb'>root</a>/<a href='/guix/log/gnu?id=a251c7890727031840b90f6f83c9016d75e99dfb'>gnu</a>/<a href='/guix/log/gnu/bootloader?id=a251c7890727031840b90f6f83c9016d75e99dfb'>bootloader</a>/<a href='/guix/log/gnu/bootloader/extlinux.scm?id=a251c7890727031840b90f6f83c9016d75e99dfb'>extlinux.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/gnu/bootloader/extlinux.scm?id=a251c7890727031840b90f6f83c9016d75e99dfb&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-08-30 08:50:44 +0200'>2022-08-30</span></td><td><a href='/guix/commit/gnu/bootloader/extlinux.scm?id=f126f23b132148192b2c9a89032a5831af0b3c52'>bootloader: Add device-tree-support? option.</a><span class='msg-avail'>...<span class='msg-tooltip'>In some specific cases where the device tree file is already loaded in RAM, it
can be preferable that the bootloader does not try to use a device tree from
the Linux kernel tree.

* gnu/bootloader.scm (&lt;bootloader-configuration&gt;)[device-tree-support?]: New field.
* gnu/bootloader/extlinux.scm (extlinux-configuration-file): Add FDTDIR line
based on &lt;device-tree-support?&gt; field of &lt;bootloader-configuration&gt;.
* doc/guix.texi (Bootloader Configuration)[device-tree-support?]: Add
documentation for the new field.
</span></span></td><td>Reza Alizadeh Majd</td></tr>