Fix incorrect way of turning CMAKE_INSTALL_LIBDIR absolute (#1261)

${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR} is not guaranteed to make sense
because CMAKE_INSTALL_LIBDIR may be specified as an absolute path, in which case
this concatenation is nonsense.

Use CMAKE_INSTALL_FULL_LIBDIR instead, which will use CMAKE_INSTALL_LIBDIR if
it's already absolute, or concatenate it onto CMAKE_INSTALL_PREFIX if it isn't.

CMakeLists.txt

@@ -392,8 +392,7 @@ unset ( DARWIN CACHE )
 unset ( MACOSX_FRAMEWORK CACHE )
 if ( CMAKE_SYSTEM MATCHES "Darwin" )
   set ( DARWIN 1 )
-  set ( CMAKE_INSTALL_NAME_DIR
-        ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR} )
+  set ( CMAKE_INSTALL_NAME_DIR ${CMAKE_INSTALL_FULL_LIBDIR} )
   if ( enable-coreaudio )
     check_include_file ( CoreAudio/AudioHardware.h COREAUDIO_FOUND )
     if ( COREAUDIO_FOUND )