frexpl: Simplify for platforms where 'long double' == 'double'.

frexpl: Simplify for platforms where 'long double' == 'double'.

* lib/frexpl.c: Include <config.h>.
(frexpl) [HAVE_SAME_LONG_DOUBLE_AS_DOUBLE]: New function.
* lib/frexp.c [!USE_LONG_DOUBLE]: Don't include <config.h> a second
time.
* m4/frexpl.m4 (gl_FUNC_FREXPL): Require gl_LONG_DOUBLE_VS_DOUBLE.
Determine FREXPL_LIBM according to HAVE_SAME_LONG_DOUBLE_AS_DOUBLE.
(gl_FUNC_FREXPL_NO_LIBM): Require gl_LONG_DOUBLE_VS_DOUBLE.
* modules/frexpl (Depends-on): Add frexp. Update conditions.
* modules/frexpl-nolibm (Depends-on): Add frexp-nolibm. Update
conditions.