Fix xsimd.pc paths

Accept both relative and absolute libdir and include dir

Fix #748

Author: serge-sans-paille

CMakeLists.txt

@@ -124,6 +124,8 @@ endif()
 # Installation
 # ============
 
+set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
+include(JoinPaths)
 include(GNUInstallDirs)
 include(CMakePackageConfigHelpers)
 
@@ -159,6 +161,8 @@ install(EXPORT ${PROJECT_NAME}-targets
         FILE ${PROJECT_NAME}Targets.cmake
         DESTINATION ${XSIMD_CMAKECONFIG_INSTALL_DIR})
 
+join_paths(libdir_for_pc_file "\${exec_prefix}" "${CMAKE_INSTALL_LIBDIR}")
+join_paths(includedir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
 configure_file(${PROJECT_NAME}.pc.in
                "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
                 @ONLY)

cmake/JoinPaths.cmake

@@ -0,0 +1,26 @@
+# This module provides function for joining paths
+# known from from most languages
+#
+# Original license:
+# SPDX-License-Identifier: (MIT OR CC0-1.0)
+# Explicit permission given to distribute this module under
+# the terms of the project as described in /LICENSE.rst.
+# 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
+# Windows not supported
+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()

xsimd.pc.in

@@ -1,6 +1,6 @@
 prefix=@CMAKE_INSTALL_PREFIX@
-libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
-includedir=${prefix}/include
+ibdir=@libdir_for_pc_file@
+includedir=@includedir_for_pc_file@
 
 Name: xsimd
 Description: xsimd provides a unified means for using SIMD features for library authors..