Disable a warning that libc++'s headers trip.

Test: make checkbuild
Bug: None
Change-Id: I117bd9ef3f4f3c8792f4b6692d7b70ad69b6922d

cc/config/clang.go

@@ -135,6 +135,9 @@ func init() {
 		// Turn off -Wthread-safety-negative, to avoid breaking projects that use -Weverything.
 		"-D_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS",
 		"-Wno-thread-safety-negative",
+
+		// libc++'s math.h has an #include_next outside of system_headers.
+		"-Wno-gnu-include-next",
 	}, " "))
 
 	pctx.StaticVariable("ClangExtraTargetCflags", strings.Join([]string{