[Bug target/63810] gcc sets incorrect macro for OS X deployment targets

[larryv at macports dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63810

--- Comment #3 from Lawrence Velázquez <larryv at macports dot org> ---
Created attachment 33968
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33968&action=edit
patch against trunk for handling deployment targets correctly

This is more of a rewrite than just a patch. I've divorced parsing of the
version string from assembly of legacy macro values from assembly of modern
macro values. It should hopefully be easier to make adjustments in the future,
since this code is more than a crude DFA implementation.