build system: make pkg-config work with includedir as absolute paths (close #890)

CMakeLists.txt

@@ -34,6 +34,12 @@ message(VERBOSE "* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
 message(VERBOSE "* Copyright (c) 2017-2022 Michele Caini <michele.caini@gmail.com>")
 message(VERBOSE "*")
 
+#
+# CMake stuff
+#
+
+list(INSERT CMAKE_MODULE_PATH 0 ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules)
+
 #
 # Compiler stuff
 #
@@ -218,8 +224,12 @@ endif()
 # Install pkg-config file
 #
 
+include(JoinPaths)
+
 set(EnTT_PKGCONFIG ${CMAKE_CURRENT_BINARY_DIR}/entt.pc)
 
+join_paths(EnTT_PKGCONFIG_INCLUDEDIR "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
+
 configure_file(
     ${EnTT_SOURCE_DIR}/cmake/in/entt.pc.in
     ${EnTT_PKGCONFIG}

cmake/in/entt.pc.in

@@ -1,5 +1,5 @@
 prefix=@CMAKE_INSTALL_PREFIX@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+includedir=@EnTT_PKGCONFIG_INCLUDEDIR@
 
 Name: EnTT
 Description: Gaming meets modern C++