inttypes, stdint: add C++11 support

inttypes, stdint: add C++11 support

C++11 says there's no need to define __STDC_CONSTANT_MACROS etc.
when including inttypes.h and stdint.h.  Support this change to
the standard.
* doc/posix-headers/inttypes.texi (inttypes.h):
* doc/posix-headers/stdint.texi (stdint.h): Document this.
* lib/inttypes.in.h (__STDC_FORMAT_MACROS) [! __cplusplus]:
Define if not defined already, for the benefit of pre-C++11 hosts.
Define the standard format macros (e.g., PRId8) always.
* lib/stdint.in.h (__STDC_CONSTANT_MACROS, __STDC_LIMIT_MACROS):
Likewise, if __cpluspus.  Define the standard constant and limit
macros (e.g., INT8_C, INT8_MAX) always.
* m4/inttypes.m4 (gl_INTTYPES_INCOMPLETE): Do not define
GL_TRIGGER_STDC_LIMIT_MACROS or __STDC_LIMIT_MACROS; no longer needed.
* m4/stdint.m4 (gl_STDINT_H): Update comments about these macros.
* tests/test-inttypes.c (__STDC_LIMIT_MACROS, __STDC_CONSTANT_MACROS)
(__STDC_FORMAT_MACROS): Do not define, since we assume C++11 API now.
* tests/test-stdint.c (__STDC_LIMIT_MACROS, __STDC_CONSTANT_MACROS):
Likewise.