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='2019-10-28 09:36:44 +0100'>2019-10-28</span></td><td><a href='/guix/commit/gnu/packages/patches/weasyprint-library-paths.patch?id=e30808ddb33f4aa0c4940ce8596ddc71ff55ed2a'>gnu: weasyprint: Add note to patch.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/patches/weasyprint-library-paths.patch: Add note.
</span></span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2019-10-23 10:05:17 +0200'>2019-10-23</span></td><td><a href='/guix/commit/gnu/packages/patches/weasyprint-library-paths.patch?id=77e0cebcadc8b96613832e1554e326d48e20f72f'>gnu: Add weasyprint.</a><span class='msg-avail'>...<span class='msg-tooltip'>Now includs the patch file missed in
73f0ed8dbfde97341716fd8b156907e4ef800873.

* gnu/packages/pdf.scm (weasyprint): New variable.
* gnu/packages/patches/weasyprint-library-paths.patch: New file.
* gnu/local.mk: Add it.
</span></span></td><td>Hartmut Goebel</td></tr>