build: cache modules from playground

Now that we are able to build our own test modules, also allow to use
cached modules so a) kernel headers are not required and b) distro
maintainers are happy.  It's still need a "--disable-test-modules" in
the configure since the default is enabled.

There's no license problems anymore since all modules come from our own
repository, we ship the sources and the modules can be easily rebuilt.
diff --git a/configure.ac b/configure.ac
index be597b4..6e67584 100644
--- a/configure.ac
+++ b/configure.ac
@@ -127,6 +127,11 @@
 	[], enable_manpages=yes)
 AM_CONDITIONAL([BUILD_MANPAGES], [test "x$enable_manpages" = "xyes"])
 
+AC_ARG_ENABLE([test-modules],
+        AS_HELP_STRING([--disable-test-modules], [disable building test modules during make check: cached modules will be used @<:@default=enabled@:>@]),
+	[], enable_test_modules=yes)
+AM_CONDITIONAL([BUILD_MODULES], [test "x$enable_test_modules" = "xyes"])
+
 AC_ARG_ENABLE([logging],
 	AS_HELP_STRING([--disable-logging], [disable system logging @<:@default=enabled@:>@]),
 	[], enable_logging=yes)
@@ -282,4 +287,5 @@
 	coverage:		${enable_coverage}
 	doc:			${enable_gtk_doc}
 	man:			${enable_manpages}
+	test-modules:           ${enable_test_modules}
 ])