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

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