isfinite, isinf, isnan tests: Avoid syntax error on MSVC 9.

isfinite, isinf, isnan tests: Avoid syntax error on MSVC 9.

* m4/math_h.m4 (gl_LONG_DOUBLE_VS_DOUBLE): New macro.
* modules/isfinite-tests (configure.ac): Require
gl_LONG_DOUBLE_VS_DOUBLE.
* modules/isinf-tests (configure.ac): Likewise.
* modules/isnan-tests (configure.ac): Likewise.
* modules/isnanl-tests (configure.ac): Likewise.
* modules/isnanl-nolibm-tests (configure.ac): Likewise.
* tests/test-isfinite.c (test_isfinitel): Avoid 80-bit long double
tests on platforms where 'long double' is the same as 'double'.
* tests/test-isinf.c (test_isinfl): Likewise.
* tests/test-isnan.c (test_long_double): Likewise.
* tests/test-isnanl.h (main): Likewise.