The kmp_os.h header is defining the PAGE_SIZE macro unconditionally,
even while it is only used directly after its definition, for the
Windows implementation of the KMP_GET_PAGE_SIZE() macro.
On at least FreeBSD, but likely all other BSDs too, this macro conflicts
with the one defined in system headers, so remove it, since nothing else
uses it. Make all Unixes use getpagesize() instead, and return 0x4000
directly for the Windows case.
(Note that Windows also has a way to retrieve page size, via the