gmacros: Only set G_ANALYZER_ANALYZING to 1 when clang static analyzer is in use

We set G_ANALYZER_ANALYZING to 1 when clang supporting static analyzing before,
but this will cause compilation error when -Werror=return-type is used and the
static analyzer is not in use because g_error static function only has
__attribute__((analyzer_noreturn)), which is useless for normal compilation.

https://bugzilla.gnome.org/show_bug.cgi?id=741901