The CMake build fails on Solaris out of the box.
* Fix Solaris vanilla CMake build; remove CI override that forces use of termcap.
* Move up pcre2_use_system_extensions to the top of CMakeLists.txt
* Use cmake_push_check_state() instead of manual management of state
* Move _GNU_SOURCE detection into pcre2_use_system_extensions, to match Autoconf
- Added CMake on z/OS to CI builds
- Added CMake module to enable _ALL_SOURCE if required for getrlimit(). This was already being done by Autoconf.
- Excluded pcre2_grep_test on EBCDIC platforms. This was already being done for Autoconf builds.
- Suppressed compiler warnings for UINT16_MAX. On most platforms it's the integer constant `65535` (a signed constant), but on z/OS and possibly AIX it's defined to `65535u` (an unsigned constant), which causes warnings about unexpected signedness in comparisons and arithmetic.
