Bug#17959689: MAKE GCC AND CLANG GIVE CONSISTENT COMPILATION WARNINGS
Bug#18313717: ENABLE -WERROR IN MAINTANER MODE WHEN COMPILING WITH CLANG
Bug#18510941: REMOVE CMAKE WORKAROUNDS FOR OLDER VERSIONS OF OS X/XCODE

Backport from mysql-trunk to mysql-5.6
(revno: 6744, 7217, 7634, 7635, 7823)