Boost does not build on OS X with either gold or binutils. The gold linker does not exist on Darwin, and binutils on Darwin provides an assembler that doesn't work for Boost.
- Introduce a variant that specifies whether to build with binutils, defaulting to true for backward compatibility
- Auto-detect whether we build on Darwin; in this case, set the gold and binutils variant defaults to false
- Clean up configure flags for as and ld
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