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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<tr><td><span title='2024-07-15 22:34:25 +0100'>2024-07-15</span></td><td><a href='/guix/commit/gnu/packages/mingw.scm?id=bf6ab0e0f5066d999e027a7eb8ecf05db71123ce'>gnu: mingw-w64-tools: Update to 12.0.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/mingw.scm (mingw-w64-tools): Update to 12.0.0.
[arguments]: Remove genlib phases.

Change-Id: Ia9060ddb1795ca2941a13435485841dbee0c1200
Signed-off-by: Christopher Baines &lt;mail@cbaines.net&gt;
</span></span></td><td>Jean-Pierre De Jesus DIAZ</td></tr>
<tr><td><span title='2024-07-15 22:34:25 +0100'>2024-07-15</span></td><td><a href='/guix/commit/gnu/packages/mingw.scm?id=21bf76c5639952ca6ee8de171739e9521a74f924'>gnu: mingw-w64-tools: Use G-Expressions.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/mingw.scm (mingw-w64-tools): Use G-Expressions.

Change-Id: I690d0aeae6c6920d2e3eb43732d6b421a3dfc59b
Signed-off-by: Christopher Baines &lt;mail@cbaines.net&gt;
</span></span></td><td>Jean-Pierre De Jesus DIAZ</td></tr>
<tr><td><span title='2024-07-15 22:34:25 +0100'>2024-07-15</span></td><td><a href='/guix/commit/gnu/packages/mingw.scm?id=17188be0f723e00377b21b767f5447d7938a116e'>gnu: make-mingw-w64: Update to 12.0.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/mingw.scm (make-mingw-w64): Update to 12.0.0.
* gnu/packages/patches/mingw-w64-6.0.0-gcc.patch: Delete patch.
* gnu/packages/patches/mingw-w64-dlltool-temp-prefix.patch: Delete patch.
* gnu/packages/patches/mingw-w64-reproducible-gendef.patch: Delete patch.
* gnu/local.mk: Unregister patches.

Change-Id: Id9d7d17926280b21016a7217da44004e06e436cd
Signed-off-by: Christopher Baines &lt;mail@cbaines.net&gt;
</span></span></td><td>Jean-Pierre De Jesus DIAZ</td></tr>
<tr><td><span title='2024-07-15 22:34:25 +0100'>2024-07-15</span></td><td><a href='/guix/commit/gnu/packages/mingw.scm?id=ce578830e6798e65f21ba713d3c1193895999286'>gnu: make-mingw-w64: Use G-Expressions.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/mingw.scm (make-mingw-w64): Use G-Expressions and
move some arguments above phases.

Change-Id: Ie095dceac635f19f146895c8e3f36d9889108a9a
Signed-off-by: Christopher Baines &lt;mail@cbaines.net&gt;
</span></span></td><td>Jean-Pierre De Jesus DIAZ</td></tr>
<tr><td><span title='2024-07-15 22:34:25 +0100'>2024-07-15</span></td><td><a href='/guix/commit/gnu/packages/mingw.scm?id=4baaa56e378ee8dd9a65858bf55e49d432d3b350'>gnu: make-mingw-w64: Memoize.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/mingw.scm (make-mingw-w64): Memoize.

Change-Id: Id5653c79e2d6268f6d8200f0f658b27169eab7c5
Signed-off-by: Christopher Baines &lt;mail@cbaines.net&gt;
</span></span></td><td>Jean-Pierre De Jesus DIAZ</td></tr>