* cleanup: remove references to no longer used stdint.h
Since 19c50b9d (Unconditionally use inttypes.h instead of trying for
stdint.h (simplification) and remove the now unnecessary inclusion in
pcre2_internal.h., 2018-11-14), stdint.h is no longer used.
Remove checks for it in autotools and CMake and document better the
expected build failures for systems that might have stdint.h (C99)
and not inttypes.h (from POSIX), like old Windows.
* cleanup: remove detection for inttypes.h which is a hard dependency
CMake checks for standard headers are not meant to be used for hard
dependencies, so will prevent a possible fallback to work.
Alternatively, the header could be checked to make the configuration
fail instead of breaking the build, but that was punted, as it was
missing anyway from autotools.
Carlo Marcelo Arenas Belón
Philip.Hazel