mesa: Add -fno-math-errno to CFLAGS.

mesa: Add -fno-math-errno to CFLAGS.

Cuts about 9k of .text size.

   text    data     bss     dec     hex filename
4992804  197808   26328 5216940  4f9aac i965_dri.so before
4983676  197808   26328 5207812  4f7704 i965_dri.so after

Also, Darwin's libm does not ever set errno, so if we care about those
systems we shouldn't rely on errno anyway.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>