w32pthreads: always use Vista+ API, drop XP support

This removes the XP compatibility code, and switches entirely to SRW
locks, which are available starting at Windows Vista.

This removes CRITICAL_SECTION use, which allows us to add
PTHREAD_MUTEX_INITIALIZER, which will be useful later.

Windows XP is hereby not a supported build target anymore.

Signed-off-by: Diego Biurrun <diego@biurrun.de>