BaseTools: Fix BaseTools C compiler flags for Xcode 5

BaseTools: Fix BaseTools C compiler flags for Xcode 5

-fno-merge-constants is not supported by clang, and it warns it will turn into
  a hard error in the future.
-Wno-deprecated-declarations removes warnings about obsolete libraries that
  are not secure.
-Wno-self-assign removes warnings from LZMA code.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anderw Fish <afish@apple.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15938 6f19259b-4bc3-4df7-8a09-765794883524