install pkgconfig file into correct path (#58)

Author: vmatare

CMakeLists.txt

@@ -53,7 +53,7 @@ if (NOT MSVC)
   set(PACKAGE_DESC "Unified Robot Description Format")
   set(pkg_conf_file "urdfdom_headers.pc")
   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 lib/pkgconfig/ COMPONENT pkgconfig)
+  install(FILES "${CMAKE_BINARY_DIR}/${pkg_conf_file}" DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig/ COMPONENT pkgconfig)
 endif()
 
 add_library(${PROJECT_NAME} INTERFACE)