[RFC][PATCH] fix math cflags overrides

[Joseph Myers]

On Thu, 24 Aug 2017, Szabolcs Nagy wrote:

> as a workaround i'd just use += instead of = in the dbl-64 makefile
> cflags definitions so at least it does not drop earlier cflags,
> the cflag definition ordering is not ideal: if i have a target
> specific sqrt, i don't want to compile it with nofma flags.

This patch is OK on the general principle of using += in Makefiles when 
overriding is not specifically intended.

For a target-specific file, the nofma flags could be worked around by just 
having a dummy e_sqrt.c and the main implementation in a separate 
e_sqrt_<arch>.c.  Or make all the cases where contraction is appropriate 
explicit (e.g. sysdeps/powerpc/fpu/e_sqrt.c makes explicit use of 
__builtin_fma to fix bug 17964).

-- 
Joseph S. Myers
joseph@codesourcery.com