Don't build gtk+:3 tests if not required

patch originally by Gilles (eva)

---
--- configure.ac
+++ configure.ac
@@ -93,9 +93,13 @@
 AC_SUBST([pkg_modules])
 PKG_CHECK_MODULES(PACKAGE, [$pkg_modules])
 
+AC_ARG_ENABLE([tests],
+	AS_HELP_STRING([--enable-tests], [Check for extra dependencies to build all tests]))
+AS_IF([test $enable_tests = yes], [
 tests_modules="gtk+-3.0 >= $REQ_GTK_VERSION"
 PKG_CHECK_MODULES(TESTS, [$tests_modules])
-
+])
+AM_CONDITIONAL([MORE_TESTS], [test $enable_tests = yes])
 GLIB_GENMARSHAL=`pkg-config --variable=glib_genmarshal glib-2.0`
 AC_SUBST(GLIB_GENMARSHAL)
 
--- tests/Makefile.am
+++ tests/Makefile.am
@@ -2,18 +2,15 @@
 	-I$(top_srcdir) \
 	$(TESTS_CFLAGS)
 
-noinst_PROGRAMS = \
+check_PROGRAMS = \
 	test-replace \
-	test-replace-widget \
 	test-server-info \
 	test-default-action \
 	test-multi-actions \
 	test-action-icons \
-	test-image \
 	test-basic \
 	test-error \
 	test-markup \
-	test-persistence \
 	test-resident \
 	test-rtl \
 	test-size-changes \
@@ -23,6 +20,22 @@
 	test-xy-actions \
 	test-xy-stress
 
+#if MORE_TESTS
+check_PROGRAMS += \
+	test-image \
+	test-replace-widget \
+	test-persistence
+
+test_replace_widget_SOURCES = test-replace-widget.c
+test_replace_widget_LDADD  = $(common_ldflags)
+
+test_image_SOURCES = test-image.c
+test_image_LDADD  = $(common_ldflags)
+
+test_persistence_SOURCES = test-persistence.c
+test_persistence_LDADD = $(common_ldflags)
+#endif
+
 common_ldflags = \
 	$(top_builddir)/libnotify/libnotify.la \
 	$(TESTS_LIBS)
@@ -30,9 +43,6 @@
 test_replace_SOURCES = test-replace.c
 test_replace_LDADD  = $(common_ldflags)
 
-test_replace_widget_SOURCES = test-replace-widget.c
-test_replace_widget_LDADD  = $(common_ldflags)
-
 test_server_info_SOURCES = test-server-info.c
 test_server_info_LDADD   = $(common_ldflags)
 
@@ -48,9 +58,6 @@
 test_action_icons_SOURCES = test-action-icons.c
 test_action_icons_LDADD  = $(common_ldflags)
 
-test_image_SOURCES = test-image.c
-test_image_LDADD  = $(common_ldflags)
-
 test_basic_SOURCES = test-basic.c
 test_basic_LDADD = $(common_ldflags)
 
@@ -75,9 +82,6 @@
 test_rtl_SOURCES = test-rtl.c
 test_rtl_LDADD = $(common_ldflags)
 
-test_persistence_SOURCES = test-persistence.c
-test_persistence_LDADD = $(common_ldflags)
-
 test_resident_SOURCES = test-resident.c
 test_resident_LDADD = $(common_ldflags)