* The -Wno-unused-but-set-variable flag is supported by gcc but not clang, so move it to a gcc-only CFLAGS.
* The -Wno-error=uninitialized flag is supported by both gcc and clang, so move it to CFLAGS shared by gcc and clang.
* Also suppress -Wunreachable-code and -Wshift-negative-value clang warnings. gcc supports -Wshift-negative-value, but only starting in gcc 6.1 and we are still using gcc 4.9 in automation.
warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-const-variable'? [-Wunknown-warning-option]
gfx/cairo/cairo/src/cairo-quartz-surface.c:1908:6: warning: code will never be executed [-Wunreachable-code]
gfx/cairo/libpixman/src/pixman-bits-image.c:268:32: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
MozReview-Commit-ID: AnQAsfDaZbk
--HG--
extra : rebase_source : 6dd94a39479e05f67f93d4e4be2bd10ece4df7be
extra : source : 34ddaea5129be2ae1e9faa0a1d905b8690909611
793bc69260