build-win32: Add root Makefile.win32

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

Makefile.win32

+default: all
+
+top_srcdir = .
+include $(top_srcdir)/Makefile.win32.common
+
+# Recursive targets
+pixman_r:
+	@$(MAKE) -C pixman -f Makefile.win32
+
+test_r:
+	@$(MAKE) -C test -f Makefile.win32
+
+clean_r:
+	@$(MAKE) -C pixman -f Makefile.win32 clean
+	@$(MAKE) -C test   -f Makefile.win32 clean
+
+# Base targets
+all: test_r
+
+clean: clean_r