Do not build tests unless required.

Gentoo: https://bugs.gentoo.org/show_bug.cgi?id=226257
--- a/test/Makefile.am	2009-04-03 16:56:51.000000000 +0200
+++ b/test/Makefile.am	2009-07-26 12:08:44.855802976 +0200
@@ -16,7 +16,7 @@
 	test-launcher-xcb			\
 	test-watch-xmessages-xcb
 
-noinst_PROGRAMS=$(XLIB_TEST) $(XCB_TEST)
+check_PROGRAMS=$(XLIB_TEST) $(XCB_TEST)
 
 test_launcher_SOURCES= test-launcher.c