floor*-ieee, ceil*-ieee, trunc*-ieee, round*-ieee: More robust checks.

floor*-ieee, ceil*-ieee, trunc*-ieee, round*-ieee: More robust checks.

* m4/floorf.m4 (gl_FUNC_FLOORF): In the test whether the function is
IEEE compliant, avoid compiler optimizations.
* m4/floor.m4 (gl_FUNC_FLOOR): Likewise.
* m4/ceilf.m4 (gl_FUNC_CEILF): Likewise.
* m4/ceil.m4 (gl_FUNC_CEIL): Likewise.
* m4/ceill.m4 (gl_FUNC_CEILL): Likewise.
* m4/truncf.m4 (gl_FUNC_TRUNCF): Likewise.
* m4/trunc.m4 (gl_FUNC_TRUNC): Likewise.
* m4/truncl.m4 (gl_FUNC_TRUNCL): Likewise.
* m4/roundf.m4 (gl_FUNC_ROUNDF): Likewise.
* m4/round.m4 (gl_FUNC_ROUND): Likewise.
* m4/roundl.m4 (gl_FUNC_ROUNDL): Likewise.