Regenerate .cabal.in files when changed

[bgamari]

One of the annoyances that I have felt with the current Make-based build system is the inability for it to rebuild ghc.cabal when ghc.cabal.in is changed. Instead one must re-run configure, which results in all sort of spurious changes, generally requiring a full rebuild. Doing this every time a module is added, removed, or renamed gets quite tiresome. For this reason I generally just resort to updating ghc.cabal by hand.

I suspect it wouldn't be that hard to teach Hadrian to do the same sort of substitution that autoconf performs, eliminating the need for a full re-configure.

[snowleopard]

I looked at ghc.cabal.in and I can see only two variables that are substituted by configure:

• @ProjectVersionMunged@
• @FFIIncludeDir@

Both values are known to Hadrian, so we can indeed do the substitution ourselves.

I think this may end up being generally useful, as there are also files like ghc-bin.cabal.in, ghci.cabal.in, prologue.txt.in, index.html.in, plus a few other that don't require a lot of substitutions.

[snowleopard]

This seems to be a duplicate of #655.