Handle cross-directory dependencies using PHONY targets and clean up
some redundancies.

On win32 the tests are built but they are not run automatically by the
build system.

A minimal 'check' target (depending on the tests being built) can
simply run them and log to the console their success/failure.

Add Makefile.win32 to the pixman root. This makefile can recursively
run the other ones to compile the library or the test suite.