build: repair the build on Windows

[compnerd]

Replicate the behaviour from upstream ICU:

#if defined(U_TIMEZONE) || defined(U_HAVE_TIMEZONE)
    /* Use the predefined value. */
#elif U_PLATFORM == U_PF_ANDROID
#   define U_TIMEZONE timezone
#elif defined(__UCLIBC__)
    // uClibc does not have __timezone or _timezone.
#elif defined(_NEWLIB_VERSION)
#   define U_TIMEZONE _timezone
#elif defined(__GLIBC__)
    // glibc
#   define U_TIMEZONE __timezone
#elif U_PLATFORM_IS_LINUX_BASED
    // not defined
#elif U_PLATFORM_USES_ONLY_WIN32_API
#   define U_TIMEZONE _timezone
#elif U_PLATFORM == U_PF_BSD && !defined(__NetBSD__)
   /* not defined */
#elif U_PLATFORM == U_PF_OS400
   /* not defined */
#elif U_PLATFORM == U_PF_IPHONE
   /* not defined */
#else
#   define U_TIMEZONE timezone
#endif

With this change, ICU at least builds, but it is unclear if it functions properly on Windows.

[compnerd]

Fixes: #15 (possibly)

[iCharlesHu]

Thank you @compnerd !!