Log Message:
x11/libxshmfence: Detect max page size correctly.

For cross-builds, whatever sysctl answers is wrong -- and even for
native builds, it may be wrong, because on some architectures with
common binary packages, the page size may vary from machine to
machine.  So use MAX_PAGE_SIZE if available, or PAGE_SIZE if not,
from machine/vmparam.h instead.

XXX Should probably also use COHERENCY_UNIT for cache line alignment,
not hard-coded 128.