The tests were including pixman.h, but pixman.h couldn't find pixman-version.h
because it was in $(top_builddir)/pixman rather than $(top_srcdir)/pixman.
noinst_PROGRAMS = $(TESTPROGRAMS)
-INCLUDES = -I$(top_srcdir)/pixman $(GTK_CFLAGS)
+INCLUDES = -I$(top_srcdir)/pixman -I$(top_builddir)/pixman $(GTK_CFLAGS)
composite_test_LDADD = $(top_builddir)/pixman/libpixman-1.la $(GTK_LIBS)
gradient_test_LDADD = $(top_builddir)/pixman/libpixman-1.la $(GTK_LIBS)