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.