Group common defines for POSIX platforms in configure and CMake scripts

Group common defines for POSIX platforms in configure and CMake scripts
(code refactoring)

* CMakeLists.txt [CMAKE_USE_PTHREADS_INIT]: Eliminate duplication of
ADD_DEFINITIONS of GC_THREADS, _REENTRANT, PARALLEL_MARK,
THREAD_LOCAL_ALLOC (and AC_MSG_WARN) for aix, cygwin, darwin, freebsd,
gnu (including kfreebsd), hpux11, irix, linux, nacl, netbsd, openbsd,
osf, solaris platforms; add comment.
* configure.ac [THREADS=posix]: Likewise.
* configure.ac [THREADS=posix]: Remove explicit hpux10 case handling.