In Darwin's hints, only add -no-cpp-precomp to ccflags if it's not an error.

In Darwin's hints, only add -no-cpp-precomp to ccflags if it's not an error.

Previously -no-cpp-precomp was added uncondtionally to cppflags and ccflags.
Apple's compiler accepts this unconditionally. gcc 4.5 warns about it, but
ignores it. gcc 4.6 treats the unknown flag as an error. Hence test whether
the flag causes problems, and only add it if it does not.

(Searching with Google suggests that this flag has been unnecessary on OS X
for some time. However, there's no clear documentation about it to confirm
when it stopped being necessary.)