Don't assume CMAKE_INSTALL_*DIR variables are relative

CMakeLists.txt

@@ -63,7 +63,7 @@ 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 "${IGN_LIB_INSTALL_DIR}/pkgconfig")
+set(ign_pkgconfig_install_dir "${CMAKE_INSTALL_FULL_LIBDIR}/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 "${ign_utilities_target}-targets.cmake")
@@ -96,9 +96,17 @@ install(
 # Configure and install the pkgconfig file (needed for utilities headers)
 file(RELATIVE_PATH
   IGN_PC_CONFIG_RELATIVE_PATH_TO_PREFIX
-  "${CMAKE_INSTALL_PREFIX}/${ign_pkgconfig_install_dir}"
+  "${ign_pkgconfig_install_dir}"
   "${CMAKE_INSTALL_PREFIX}"
 )
+
+# TODO(jrivero): CMake 3.20 provides cmake_path(APPEND) which implements the
+# same functionality as join_paths(). Remove JoinPaths in the next major version
+# if all the supported platforms are in 3.20 version.
+include(JoinPaths)
+join_paths(IGN_PC_LIBDIR "\${prefix}" "${CMAKE_INSTALL_LIBDIR}")
+join_paths(IGN_PC_INCLUDEDIR "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}" "${IGN_INCLUDE_INSTALL_DIR_POSTFIX}")
+
 configure_file(${ign_pkgconfig_input} ${ign_pkgconfig_output} @ONLY)
 
 install(

cmake/IgnPackaging.cmake

@@ -121,16 +121,16 @@ macro(ign_setup_packages)
     # (but later on when installing)
     set(CMAKE_BUILD_WITH_INSTALL_RPATH FALSE)
 
-    set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/${IGN_LIB_INSTALL_DIR}")
+    set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_FULL_LIBDIR}")
 
     # add the automatically determined parts of the RPATH
     # which point to directories outside the build tree to the install RPATH
     set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
 
     # the RPATH to be used when installing, but only if its not a system directory
-    list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${CMAKE_INSTALL_PREFIX}/${IGN_LIB_INSTALL_DIR}" isSystemDir)
+    list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${CMAKE_INSTALL_FULL_LIBDIR}" isSystemDir)
     if("${isSystemDir}" STREQUAL "-1")
-      set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/${IGN_LIB_INSTALL_DIR}")
+      set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_FULL_LIBDIR}")
     endif("${isSystemDir}" STREQUAL "-1")
   endif()
 
@@ -198,13 +198,17 @@ function(_ign_create_pkgconfig)
   endif()
 
   set(pkgconfig_output "${CMAKE_BINARY_DIR}/cmake/pkgconfig/${target_name}.pc")
-  set(pkgconfig_install_dir "${IGN_LIB_INSTALL_DIR}/pkgconfig")
+  set(pkgconfig_install_dir "${CMAKE_INSTALL_FULL_LIBDIR}/pkgconfig")
   file(RELATIVE_PATH
     PC_CONFIG_RELATIVE_PATH_TO_PREFIX
-    "${CMAKE_INSTALL_PREFIX}/${pkgconfig_install_dir}"
+    "${pkgconfig_install_dir}"
     "${CMAKE_INSTALL_PREFIX}"
   )
 
+  include("${IGNITION_CMAKE_DIR}/JoinPaths.cmake")
+  join_paths(PC_LIBDIR "\${prefix}" "${CMAKE_INSTALL_LIBDIR}")
+  join_paths(PC_INCLUDEDIR "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}" "${IGN_INCLUDE_INSTALL_DIR_POSTFIX}")
+
   configure_file(${pkgconfig_input} ${pkgconfig_output} @ONLY)
 
   install(

cmake/JoinPaths.cmake

@@ -0,0 +1,22 @@
+# This module provides function for joining paths
+# known from most languages
+#
+# SPDX-License-Identifier: (MIT OR CC0-1.0)
+# Copyright 2020 Jan Tojnar
+# https://github.com/jtojnar/cmake-snips
+#
+# Modelled after Python’s os.path.join
+# https://docs.python.org/3.7/library/os.path.html#os.path.join
+function(join_paths joined_path first_path_segment)
+  set(temp_path "${first_path_segment}")
+  foreach(current_segment IN LISTS ARGN)
+    if(NOT ("${current_segment}" STREQUAL ""))
+      if(IS_ABSOLUTE "${current_segment}")
+        set(temp_path "${current_segment}")
+      else()
+        set(temp_path "${temp_path}/${current_segment}")
+      endif()
+    endif()
+  endforeach()
+  set(${joined_path} "${temp_path}" PARENT_SCOPE)
+endfunction()

cmake/pkgconfig/ignition-component.pc.in

@@ -1,6 +1,6 @@
 prefix=${pcfiledir}/@PC_CONFIG_RELATIVE_PATH_TO_PREFIX@
-libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@/@IGN_INCLUDE_INSTALL_DIR_POSTFIX@
+libdir=@PC_LIBDIR@
+includedir=@PC_INCLUDEDIR@
 
 Name: Ignition @GZ_DESIGNATION@ @component_name@
 Description: A set of @GZ_DESIGNATION@ @component_name@ classes for robot applications

cmake/pkgconfig/ignition.pc.in

@@ -1,6 +1,6 @@
 prefix=${pcfiledir}/@PC_CONFIG_RELATIVE_PATH_TO_PREFIX@
-libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@/@IGN_INCLUDE_INSTALL_DIR_POSTFIX@
+libdir=@PC_LIBDIR@
+includedir=@PC_INCLUDEDIR@
 
 Name: Ignition @GZ_DESIGNATION@
 Description: A set of @GZ_DESIGNATION@ classes for robot applications

config/ignition-cmake.pc.in

@@ -1,5 +1,5 @@
 prefix=${pcfiledir}/@IGN_PC_CONFIG_RELATIVE_PATH_TO_PREFIX@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@/@IGN_INCLUDE_INSTALL_DIR_POSTFIX@
+includedir=@IGN_PC_INCLUDEDIR@
 
 Name: Ignition @GZ_DESIGNATION@
 Description: Build system package for the ignition libraries