cmake: respect configuration of CMAKE_INSTALL_RPATH

This will allow users to set this to a value more appropriate for their
installation.

This would be useful for us at Homebrew, for example, where we set this
to the appropriate value for Homebrew installations.[^1]

[^1]: https://github.com/Homebrew/homebrew-core/blob/a661bd4800424c1bfe7715d1c72c6566545cc2d6/Formula/mapserver.rb#L52

CMakeLists.txt

@@ -234,8 +234,10 @@ if(NOT DEFINED CMAKE_INSTALL_LIBDIR)
   set(CMAKE_INSTALL_LIBDIR "${_LIBDIR_DEFAULT}" CACHE PATH "object code libraries (${_LIBDIR_DEFAULT})")
 endif()
 
-SET(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
-SET(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
+if(NOT DEFINED CMAKE_INSTALL_RPATH)
+  SET(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
+  SET(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
+endif()
 SET(CMAKE_MACOSX_RPATH ON)
 
 if(LINK_STATIC_LIBMAPSERVER)