Ensure relocatable config files (#419)

* fix: Use PACKAGE_PREFIX_DIR to ensure relocatable CMake install for conan

Signed-off-by: Stefan Profanter <stefan.profanter@agile-robots.com>

* Compute relative path for pkgconfig folder.
  This accounts for arch-dependent lib folders.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>
Co-authored-by: Steve Peters <scpeters@openrobotics.org>
Co-authored-by: Jose Luis Rivero <jrivero@osrfoundation.org>

CMakeLists.txt

@@ -304,10 +304,16 @@ else (build_errors)
 
   ########################################
   # Make the package config file
+  set(PC_CONFIG_INSTALL_DIR ${LIB_INSTALL_DIR}/pkgconfig)
+  file(RELATIVE_PATH
+    PC_CONFIG_RELATIVE_PATH_TO_PREFIX
+    "${CMAKE_INSTALL_PREFIX}/${PC_CONFIG_INSTALL_DIR}"
+    "${CMAKE_INSTALL_PREFIX}"
+  )
   configure_file(${CMAKE_SOURCE_DIR}/cmake/sdformat_pc.in
     ${CMAKE_CURRENT_BINARY_DIR}/cmake/${PROJECT_NAME_LOWER}.pc @ONLY)
   install(FILES ${CMAKE_CURRENT_BINARY_DIR}/cmake/${PROJECT_NAME_LOWER}.pc DESTINATION
-    ${LIB_INSTALL_DIR}/pkgconfig COMPONENT pkgconfig)
+    ${PC_CONFIG_INSTALL_DIR} COMPONENT pkgconfig)
 
   ########################################
   # Configure documentation uploader

cmake/sdf_config.cmake.in

@@ -17,14 +17,14 @@ if(NOT TARGET @sdf_import_target_name@)
   include("${CMAKE_CURRENT_LIST_DIR}/@sdf_target_output_filename@")
 endif()
 
-list(APPEND @PKG_NAME@_INCLUDE_DIRS "@CMAKE_INSTALL_PREFIX@/include/sdformat-@SDF_VERSION@")
+list(APPEND @PKG_NAME@_INCLUDE_DIRS "${PACKAGE_PREFIX_DIR}/include/sdformat-@SDF_VERSION@")
 
-list(APPEND @PKG_NAME@_CFLAGS "-I@CMAKE_INSTALL_PREFIX@/include/sdformat-@SDF_VERSION@")
+list(APPEND @PKG_NAME@_CFLAGS "-I${PACKAGE_PREFIX_DIR}/include/sdformat-@SDF_VERSION@")
 if (NOT WIN32)
   list(APPEND @PKG_NAME@_CXX_FLAGS "${@PKG_NAME@_CFLAGS} -std=c++17")
 endif()
 
-list(APPEND @PKG_NAME@_LIBRARY_DIRS "@CMAKE_INSTALL_PREFIX@/@CMAKE_INSTALL_LIBDIR@")
+list(APPEND @PKG_NAME@_LIBRARY_DIRS "${PACKAGE_PREFIX_DIR}/@CMAKE_INSTALL_LIBDIR@")
 
 set(@PKG_NAME@_LIBRARIES @sdf_import_target_name@)
 
@@ -37,4 +37,4 @@ find_package(ignition-math@IGN_MATH_VER@)
 list(APPEND @PKG_NAME@_INCLUDE_DIRS ${IGNITION-MATH_INCLUDE_DIRS})
 list(APPEND @PKG_NAME@_LIBRARY_DIRS ${IGNITION-MATH_LIBRARY_DIRS})
 
-list(APPEND @PKG_NAME@_LDFLAGS "-L@CMAKE_INSTALL_PREFIX@/@LIB_INSTALL_DIR@")
+list(APPEND @PKG_NAME@_LDFLAGS "-L${PACKAGE_PREFIX_DIR}/@LIB_INSTALL_DIR@")

cmake/sdformat_pc.in

@@ -1,4 +1,4 @@
-prefix="@CMAKE_INSTALL_PREFIX@"
+prefix=${pcfiledir}/@PC_CONFIG_RELATIVE_PATH_TO_PREFIX@
 libdir=${prefix}/@LIB_INSTALL_DIR@
 includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@