Fix assumption that CMAKE_INSTALL_*DIR paths are relative.

This solution uses relative paths if possible, allowing the package to be
relocatable, but still works correctly if CMAKE_INSTALL_*DIR paths are absolute.

CMakeLists.txt

@@ -35,6 +35,11 @@ else()
 endif()
 string(REGEX REPLACE "[^/]+" ".." RELATIVE_PATH_CMAKE_DIR_TO_PREFIX "${CMAKE_CONFIG_INSTALL_DIR}")
 
+include(cmake/JoinPaths.cmake)
+join_paths(cmake_conf_include_dirs
+  "\${${PROJECT_NAME}_DIR}/${RELATIVE_PATH_CMAKE_DIR_TO_PREFIX}"
+   "${CMAKE_INSTALL_INCLUDEDIR}")
+
 set(PACKAGE_NAME ${PROJECT_NAME})
 set(cmake_conf_file "${PROJECT_NAME}-config.cmake")
 configure_file("${CMAKE_CURRENT_SOURCE_DIR}/cmake/${cmake_conf_file}.in" "${CMAKE_BINARY_DIR}/${cmake_conf_file}" @ONLY)
@@ -57,6 +62,7 @@ install(FILES
 if (NOT MSVC)
   set(PACKAGE_DESC "Unified Robot Description Format")
   set(pkg_conf_file "urdfdom_headers.pc")
+  join_paths(pkg_conf_includedir "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
   configure_file("${CMAKE_CURRENT_SOURCE_DIR}/cmake/pkgconfig/${pkg_conf_file}.in" "${CMAKE_BINARY_DIR}/${pkg_conf_file}" @ONLY)
   install(FILES "${CMAKE_BINARY_DIR}/${pkg_conf_file}" DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig/ COMPONENT pkgconfig)
 endif()

cmake/JoinPaths.cmake

@@ -0,0 +1,22 @@
+# This module provides a 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/urdfdom_headers.pc.in

@@ -1,7 +1,7 @@
 # This file was generated by CMake for @PROJECT_NAME@
 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+includedir=@pkg_conf_includedir@
 
 Name: @PACKAGE_NAME@
 Description: @PACKAGE_DESC@

cmake/urdfdom_headers-config.cmake.in

@@ -3,7 +3,7 @@ if (@PACKAGE_NAME@_CONFIG_INCLUDED)
 endif()
 set(@PACKAGE_NAME@_CONFIG_INCLUDED TRUE)
 
-set(@PACKAGE_NAME@_INCLUDE_DIRS "${@PROJECT_NAME@_DIR}/@RELATIVE_PATH_CMAKE_DIR_TO_PREFIX@/@CMAKE_INSTALL_INCLUDEDIR@")
+set(@PACKAGE_NAME@_INCLUDE_DIRS "@cmake_conf_include_dirs@")
 
 include("${@PACKAGE_NAME@_DIR}/@[email protected]")