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
 )
 
 ##

cmake/pkg-config.pc.in

@@ -1,4 +1,7 @@
-Name: ${PROJECT_NAME}
+prefix=@CMAKE_INSTALL_PREFIX@
+includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+
+Name: @PROJECT_NAME@
 Description: JSON for Modern C++
-Version: ${PROJECT_VERSION}
+Version: @PROJECT_VERSION@
-Cflags: -I${CMAKE_INSTALL_FULL_INCLUDEDIR}
+Cflags: -I${includedir}