Handle absolute include and lib paths in cmake

Author: madebr

CMakeLists.txt

@@ -1194,6 +1194,17 @@ if(SDLMIXER_INSTALL)
       set(SDL_PKGCONFIG_PREFIX "${CMAKE_INSTALL_PREFIX}")
     endif()
 
+    if(IS_ABSOLUTE "${CMAKE_INSTALL_INCLUDEDIR}")
+      set(INCLUDEDIR_FOR_PKG_CONFIG "${CMAKE_INSTALL_INCLUDEDIR}")
+    else()
+      set(INCLUDEDIR_FOR_PKG_CONFIG "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
+    endif()
+    if(IS_ABSOLUTE "${CMAKE_INSTALL_LIBDIR}")
+      set(LIBDIR_FOR_PKG_CONFIG "${CMAKE_INSTALL_LIBDIR}")
+    else()
+      set(LIBDIR_FOR_PKG_CONFIG "\${prefix}/${CMAKE_INSTALL_LIBDIR}")
+    endif()
+
     string(JOIN " " PC_REQUIRES ${PC_REQUIRES})
     string(JOIN " " PC_LIBS ${PC_LIBS})
     configure_file(cmake/sdl3-mixer.pc.in sdl3-mixer.pc @ONLY)

cmake/sdl3-mixer.pc.in

@@ -1,7 +1,7 @@
 prefix=@SDL_PKGCONFIG_PREFIX@
 exec_prefix=${prefix}
-libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+libdir=@LIBDIR_FOR_PKG_CONFIG@
+includedir=@INCLUDEDIR_FOR_PKG_CONFIG@
 
 Name: @PROJECT_NAME@
 Description: mixer library for Simple DirectMedia Layer