Distribution.Simple.Build.Macros

Generate cabal_macros.h - CPP macros for package version testing

When using CPP you get

VERSION_<package>
MIN_VERSION_<package>(A,B,C)

for each package in build-depends, which is true if the version of package in use is >= A.B.C, using the normal ordering on version numbers.

TODO Figure out what to do about backpack and internal libraries. It is very suspicious that this stuff works with munged package identifiers