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

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

* lib/printf-frexpl.c: Include <config.h>.
(printf_frexpl) [HAVE_SAME_LONG_DOUBLE_AS_DOUBLE]: New function.
* lib/printf-frexp.c [!USE_LONG_DOUBLE]: Don't include <config.h> a
second time.
* m4/printf-frexpl.m4 (gl_FUNC_PRINTF_FREXPL): Require
gl_LONG_DOUBLE_VS_DOUBLE.
* modules/printf-frexpl (Depends-on): Add printf-frexp. Update
conditions.