gnu: rapidjson: Fix FTBFS with GCC 7.

* gnu/packages/patches/rapidjson-gcc-compat.patch: New file.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.
* gnu/packages/web.scm (rapidjson)[source](patches): New field.

gnu/local.mk

@@ -1240,6 +1240,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/qtwebkit-pbutils-include.patch		\
   %D%/packages/patches/randomjungle-disable-static-build.patch	\
   %D%/packages/patches/rapicorn-isnan.patch			\
+  %D%/packages/patches/rapidjson-gcc-compat.patch		\
   %D%/packages/patches/raptor2-heap-overflow.patch		\
   %D%/packages/patches/ratpoints-sturm_and_rp_private.patch	\
   %D%/packages/patches/ratpoison-shell.patch			\

gnu/packages/patches/rapidjson-gcc-compat.patch

@@ -0,0 +1,18 @@
+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

gnu/packages/web.scm

@@ -761,6 +761,7 @@ current version of any major web browser.")
               (sha256
                (base32
                 "1jixgb8w97l9gdh3inihz7avz7i770gy2j2irvvlyrq3wi41f5ab"))
+              (patches (search-patches "rapidjson-gcc-compat.patch"))
               (modules '((guix build utils)))
               (snippet
                '(begin