cmake: Generate and install a pkg-config file.

The meson builds install a pkg-config file, but the cmake builds don't.
This adds a pkg-config file to the cmake builds that is functionally
equivalent to the one generated from meson.

cmake/pkg-config.pc.in

@@ -0,0 +1,4 @@
+Name: ${PROJECT_NAME}
+Description: ${PROJECT_DESCRIPTION}
+Version: ${PROJECT_VERSION}
+Cflags: -I${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}