This flag adds specific support for the -arch and -isysroot flags of GCC
on MacOSX 10.4 or later.
The patch consists of two parts:
1) Remove these flags (and their arguments) from the base CFLAGS/
LDFLAGS when compiling extensions on OSX 10.3 or earlier because GCC
doesn't support those arguments in the version of GCC that is shipped
what the version of the OS.
2) Strip -arch and -isysroot (again including their arguments) from the
base CFLAGS/LDFLAGS when the user has specified new values for them
in the extra_compile_args and extra_link args.
The second part is needed because -isysroot can only be specified once
and the -arch option is incremental, without this patch you cannot
compile using a different SDK or for fewer architectures.
A reason for wanting to do the latter is software like psyco that is only
fully supported on one of the architectures for OSX.