Improve flexibility of dependencies locations on OS X

- Replaced @executable_path by @rpath for more flexibility
 - Updated freetype and sndfile libs as follow:

    install_name_tool -id "@rpath/../Frameworks/freetype.framework/Versions/A/freetype" freetype
    install_name_tool -id "@rpath/../Frameworks/sndfile.framework/Versions/A/sndfile" sndfile

CMakeLists.txt

@@ -232,7 +232,7 @@ else()
     # NOTE: it's not required to link agains SFML.framework
     set_target_properties(SFML PROPERTIES
                           BUILD_WITH_INSTALL_RPATH 1
-                          INSTALL_NAME_DIR "@executable_path/../Frameworks")
+                          INSTALL_NAME_DIR "@rpath")
 
     # install rule
     install(TARGETS SFML

cmake/Macros.cmake

@@ -84,7 +84,7 @@ macro(sfml_add_library target)
         # adapt install directory to allow distributing dylibs/frameworks in user’s frameworks/application bundle
         set_target_properties(${target} PROPERTIES
                               BUILD_WITH_INSTALL_RPATH 1
-                              INSTALL_NAME_DIR "@executable_path/../Frameworks")
+                              INSTALL_NAME_DIR "@rpath")
     endif()
 
     # enable automatic reference counting on iOS