Patches for better Darwin support

[Shantonu Sen]

Hello,

The following patches add:
1) Identification for Darwin x86 (i386-apple-darwin)
2) Add -no-cpp-precomp for Darwin, since the compiler falls back to not 
using cpp-precomp anyway. The patch will speed up compiles by not even 
trying to use cpp-precomp
3) Don't like against the heavy-weight Carbon framework if 
--without-carbon was specified

Shantonu Sen

configure.in.diff
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darwin.h.diff
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