ports/177224: building www/firefox fails

[Tsurutani Naoki]

building with lang/clang-devel fails (*1).
building with lang/clang successfully finishes, but firefox crashes
 with segmentation fault just after running it.
building without clang successfully finishes, and no errors.
I want a knob to choice the compiler.

(*1) __attribute__((used)) might be ignored with clang-devel (clang 3.3) ?
using [[gnu::used]] instead of above failed:
% make
...
/usr/local/work/usr/ports/www/firefox/work/mozilla-
release/xpcom/reflect/xptcall/src/md/unix/xptcinvoke_gcc_x86_unix.cpp:12:13: error:
      '__used__' attribute cannot be applied to types
static void ATTRIBUTE_USED __attribute__ ((regparm(3)))
            ^
../../../../../../../xpcom/reflect/xptcall/src/md/unix/xptc_gcc_x86_unix.h:19:26: note:
      expanded from macro 'ATTRIBUTE_USED'
#define ATTRIBUTE_USED [[gnu::__used__]]
                         ^
1 error generated.
%