This reverts commit d393c6c538.
The commit removed C++11 compatibility on darwin by overriding the
--std=c++0x flag in CXXFLAGS. Which lead to a failing build of mapnik,
which depends on the move constructors being available in the icu-lib.
Since it builds fine without the headerpad_max_install_names flag, we
simply undo the change that introduced this flag.
334a1a6f8a