[patch] Add -static-libquadmath option

[FX]

Version 2 of the patch, now handling the darwin case (thanks Iain) and expressely noting in the documentation the implications on redistribution (thanks Joseph).
Bootstrapped and regtested on x86_64-apple-darwin14.

OK to commit?

I need a C/driver options maintainer, or global reviewer, to OK the C changes (but they really are obvious).
As Iain suggested the darwin.h change, I consider it pre-approved by him :)




> We have a -static-libgfortran option, but on targets where we support quad-prec math through libquadmath, we didn’t have an equivalent -static-libquadmath so far. This patch adds it, in what I think is a rather straightforward manner.
> 
> The only minor complication comes from the fact that previously, linking libquadmath was conditionally through libgfortran.spec. So the spec was modified: when we use -static-libquadmath, the driver itself includes the -lquadmath (surrounded by the necessary static linking directives), so that libgfortran.spec shouldn’t do it again.
> 
> Bootstrapped and regtested on x86_64 linux. OK to commit?


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