Compute path to prefix for ignition-cmake.pc

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

CMakeLists.txt

@@ -56,6 +56,7 @@ set(ign_version_output "${PROJECT_NAME_LOWER}-config-version.cmake")
 set(ign_config_install_dir "${CMAKE_INSTALL_DATAROOTDIR}/cmake/${PROJECT_NAME_LOWER}")
 set(ign_pkgconfig_input "${CMAKE_CURRENT_SOURCE_DIR}/config/ignition-cmake.pc.in")
 set(ign_pkgconfig_output "${CMAKE_BINARY_DIR}/ignition-cmake${PROJECT_VERSION_MAJOR}.pc")
+set(ign_pkgconfig_install_dir "${IGN_LIB_INSTALL_DIR}/pkgconfig")
 set(ign_utilities_target ${PROJECT_EXPORT_NAME}-utilities)
 set(ign_utilities_import_target_name ${PROJECT_EXPORT_NAME}::${ign_utilities_target})
 set(ign_utilities_target_output_filename "${ign_utilities_target}-targets.cmake")
@@ -86,11 +87,16 @@ install(
 
 #--------------------------------------
 # Configure and install the pkgconfig file (needed for utilities headers)
+file(RELATIVE_PATH
+  IGN_PC_CONFIG_RELATIVE_PATH_TO_PREFIX
+  "${CMAKE_INSTALL_PREFIX}/${ign_pkgconfig_install_dir}"
+  "${CMAKE_INSTALL_PREFIX}"
+)
 configure_file(${ign_pkgconfig_input} ${ign_pkgconfig_output} @ONLY)
 
 install(
   FILES ${ign_pkgconfig_output}
-  DESTINATION ${IGN_LIB_INSTALL_DIR}/pkgconfig
+  DESTINATION ${ign_pkgconfig_install_dir}
   COMPONENT pkgconfig)
 
 #============================================================================

config/ignition-cmake.pc.in

@@ -1,4 +1,4 @@
-prefix=@CMAKE_INSTALL_PREFIX@
+prefix=${pcfiledir}/@IGN_PC_CONFIG_RELATIVE_PATH_TO_PREFIX@
 includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@/ignition/@IGN_DESIGNATION@@PROJECT_VERSION_MAJOR@
 
 Name: Ignition @IGN_DESIGNATION@