stdint: detect good enough pre-C++11 stdint.h in C++ mode
When gnulib is configured in C++ mode for a system with a working C99
implementation of stdint.h that predates C++11, gnulib ends up
substituting stdint.h anyway. This works on most targets, but on e.g.,
64-bit MinGW, it doesn't, as gnulib's substitute assumes LP64, while
MinGW is LLP64. Instead of trying to detect the right types, detect
good-enough-pre-C++11 stdint.h and in such case define
__STDC_CONSTANT_MACROS/__STDC_LIMIT_MACROS in config.h.
* m4/stdint.m4 (gl_STDINT_H): Always define __STDC_CONSTANT_MACROS
/ __STDC_LIMIT_MACROS while checking whether the system stdint.h
conforms to C99. If it does, check whether it hides symbols
behind the __STDC_{CONSTANT|LIMIT}_MACROS macros. Then if it
does, define those macros in config.h.