CMake: generate a pkg-config file that follow pkg-config conventions (#4456)

* CMake: configure pkg-config with @only

And use @ replacement instead of {}, since ${} is significant in
pkg-config's format

* CMake: use conventional prefix and incluedir variables in pkg-config

This is more standard, and has some advantages when cross compiling.
This also means that the pkg-config files generated by Meson and CMake
now match.

CMakeLists.txt

@@ -149,6 +149,7 @@ endif()
 CONFIGURE_FILE(
     "${CMAKE_CURRENT_SOURCE_DIR}/cmake/pkg-config.pc.in"
     "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
+    @ONLY
 )
 
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