Build: Include the CMake files in the distribution.

This was supposed to be done in 2020 with 5.2.5 release
already but it was noticed only today. 5.2.5 and 5.2.6
even mention experiemental CMake support in the NEWS entries.

Thanks to Olivier B. for reporting the problem.

Makefile.am

@@ -52,6 +52,8 @@ EXTRA_DIST = \
 	dos \
 	windows \
 	macosx \
+	cmake \
+	CMakeLists.txt \
 	autogen.sh \
 	Doxyfile.in \
 	COPYING.GPLv2 \