[m-rev.] diff: fix GCC labels with GCC 4.9 on x86_64

[Zoltan Somogyi]

On Thu, 19 Feb 2015 09:14:43 +1100 (AEDT), Julien Fischer <jfischer at opturion.com> wrote:
>      Disable GCC's -ftree-dominator-opts optimization when using
>      GCC 4.9 on the x86_64 and x86.  As with GCC 4.8, its use seems
>      incompatible with our use of GCC labels.

Wouldn't it be safer to check whether the C compiler accepts
-ftree-dominator-opts, and disable the option if it does?
That way, we won't have to repeat this for 4.10, 4.11 etc.
If the option does respect our use of gcc labels in some
gcc versions, then those versions could be whitelisted,
but effectively whitelisting unknown future versions
seems unwise.

Zoltan.