diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4e58a931..9b050a0d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -458,8 +458,16 @@ add_subdirectory ( doc )
 # pkg-config support
 set ( prefix "${CMAKE_INSTALL_PREFIX}" )
 set ( exec_prefix "\${prefix}" )
-set ( libdir "\${exec_prefix}/${LIB_INSTALL_DIR}" )
-set ( includedir "\${prefix}/${INCLUDE_INSTALL_DIR}" )
+if ( IS_ABSOLUTE "${LIB_INSTALL_DIR}" )
+  set ( libdir "${LIB_INSTALL_DIR}" )
+else ()
+  set ( libdir "\${exec_prefix}/${LIB_INSTALL_DIR}" )
+endif ()
+if ( IS_ABSOLUTE "${INCLUDE_INSTALL_DIR}" )
+  set ( includedir "${INCLUDE_INSTALL_DIR}" )
+else ()
+  set ( includedir "\${prefix}/${INCLUDE_INSTALL_DIR}" )
+endif ()
 configure_file ( fluidsynth.pc.in
 	${CMAKE_BINARY_DIR}/fluidsynth.pc IMMEDIATE @ONLY )
 install ( FILES ${CMAKE_BINARY_DIR}/fluidsynth.pc