Do not use the system wctype() on Windows

[kevinushey]

On Windows, HAVE_ISWCTYPE and HAVE_WCTYPE are defined (both in win32/config.h and by configure under MinGW), so TRE_USE_SYSTEM_WCTYPE is enabled. But the Windows CRT's wctype() has no "blank" class -- wctype("blank") returns 0, so a pattern using [[:blank:]] fails to compile with REG_ECTYPE. The CRT's iswprint(L'\t') also incorrectly returns true, so [[:print:]] matches a tab.

TRE already carries fixes for both in the !TRE_USE_SYSTEM_WCTYPE path -- tre_isblank_func(), and the Windows tab check in tre_isprint_func() imported from #106 -- but they are currently unreachable on Windows because the system-wctype path is selected instead.

This excludes _WIN32 from the TRE_USE_SYSTEM_WCTYPE condition so that TRE's own class-function table is used there, making those existing workarounds effective. R's bundled copy of TRE has shipped this opt-out for many years.

No change on other platforms; make check passes on macOS.