isinf-no-c++: New module.

isinf-no-c++: New module.

Reported by G. Branden Robinson <g.branden.robinson@gmail.com> in
<https://lists.gnu.org/archive/html/bug-gnulib/2025-12/msg00150.html>.

* modules/isinf-no-c++: New file, based on modules/isinf.
* modules/isinf: Essentially an alias of isinf-no-c++ now.
* modules/isinf-no-c++-tests: Renamed from modules/isinf-tests.
* lib/math.in.h (isinf): Test also GNULIB_ISINF_NO_CXX.
* m4/math_h.m4 (gl_MATH_H_REQUIRE_DEFAULTS): Initialize
GNULIB_ISINF_NO_CXX.
* modules/math-h (Makefile.am): Substitute GNULIB_ISINF_NO_CXX.
* modules/fmod (Depends-on): Add isinf-no-c++. Remove isinf.
* modules/fmodl (Depends-on): Likewise.
* modules/hypot (Depends-on): Likewise.
* modules/hypotl (Depends-on): Likewise.
* modules/modf (Depends-on): Likewise.
* modules/modfl (Depends-on): Likewise.
* modules/remainder (Depends-on): Likewise.
* modules/remainderf (Depends-on): Likewise.
* modules/remainderl (Depends-on): Likewise.
* modules/strtod (Depends-on): Likewise.
* modules/strtof (Depends-on): Likewise.
* modules/strtold (Depends-on): Likewise.