Build: Silence shift-negative-value warning

Pixman's headers include a representation of -1 in fixed-point, which is
-1 << 16. This trips a GCC warning about shifting negative values. As we
can't do much about it, just silence the warning.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>

configure.ac

@@ -547,7 +547,8 @@ AM_CONDITIONAL(BUILD_SETBACKLIGHT, test "x$enable_setbacklight" = "xyes")
 
 if test "x$GCC" = "xyes"; then
 	GCC_CFLAGS="-Wall -Wextra -Wno-unused-parameter \
-		-Wno-missing-field-initializers -g -fvisibility=hidden \
+		-Wno-shift-negative-value -Wno-missing-field-initializers \
+		-g -fvisibility=hidden \
 		-Wstrict-prototypes -Wmissing-prototypes -Wsign-compare"
 fi
 AC_SUBST(GCC_CFLAGS)