Also install a pkg-config file

This for interoperability with other build systems

cmake/godot-cpp.pc.in

@@ -0,0 +1,10 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
+includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
+
+Name: godot-cpp
+Description: C++ bindings for the Godot script API
+Version: @GODOT_API_VERSION@
+Libs: -L${libdir} -l@GODOTCPP_OUTPUT_NAME@
+Cflags: -I${includedir}

cmake/godotcpp.cmake

@@ -287,5 +287,18 @@ function( godotcpp_installable )
             DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/godot"
             COMPONENT "godot-cpp"
         )
+
+        # Install the pkg-config file
+        get_target_property(GODOTCPP_OUTPUT_NAME "${PROJECT_NAME}" OUTPUT_NAME)
+        configure_file(
+            "${CMAKE_CURRENT_SOURCE_DIR}/cmake/godot-cpp.pc.in"
+            "${CMAKE_CURRENT_BINARY_DIR}/godot-cpp.pc"
+            @ONLY
+        )
+        install(
+            FILES "${CMAKE_CURRENT_BINARY_DIR}/godot-cpp.pc"
+            DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig"
+            COMPONENT "godot-cpp"
+        )
     endif()
 endfunction()