Add the JSON_* definitions to the CMake pkg-config file

The installed pkg-config file only had the include path, so a
pkg-config user of a library installed with, for example,
-DJSON_Diagnostics=ON compiled without JSON_DIAGNOSTICS=1, unlike users
of the CMake target. Append the same definitions the target has. For the
default options, the file is unchanged.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

cmake/pkg-config.pc.in

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