Include dist/build/autogen dir in the -I search list for cabal sandbox installations

[roman]

This in order for libraries with C dependencies to be built with cabal-sandbox

This issue was discovered in a conversation with @dcoutts on #haskell IRC channel which I'll replicate over here, to add some context to the ticket:

romanandreg: dcoutts_: master branch of cabal, I'm trying to install HsOpenSSL through a sandbox, can't find cabal_macros.h file on cbits/HsOpenSSL.h

romanandreg: dcoutts_: when installing without a sandbox, works perfectly

romanandreg: dcoutts_: Gist for + info https://gist.github.com/roman/5681021

dcoutts_: romanandreg: using same version of cabal-install in both cases?

romanandreg: yes, it was

dcoutts_: romanandreg: of course cbits/HsOpenSSL.h should not be trying to #include that file in the first place, because it's hard coding the dist dir location

dcoutts_: romanandreg: indeed that might be the issue, I'm not sure if the sandbox changes the dist dir

dcoutts_: romanandreg: should be easy to check, does dist exist in that dir?

dcoutts_: romanandreg: oh you're installing a tarball so you can't check

dcoutts_: romanandreg: ah yes there we go, dist/dist-sandbox-dc2d92ee

dcoutts_: romanandreg: so the bug is in the HsOpenSSL package because it's hard coding ../dist/build/autogen/cabal_macros.h

dcoutts_: romanandreg: but the location of the dist dir is not known to the package

romanandreg: dcoutts_: ohh…

romanandreg: dcoutts_: so basically hack the HsOpenSSL project to not rely on the fixed path of dist

dcoutts_: romanandreg: yes

dcoutts_: romanandreg: in principle it is currently impossible

dcoutts_: romanandreg: but it sounds like it'd be a useful feature to include the dist/build/autogen dir in the -I search list

dcoutts_: romanandreg: for .c files I mean

[23Skidoo]

So we want to always pass $DIST_PREF/build/autogen in the -I search list. OK, should be easy to fix.

[dcoutts]

Yes, easy. Look at the ghcOptCppIncludePath in componentGhcOptions vs componentCcGhcOptions:

      ghcOptCppIncludePath  = [autogenModulesDir lbi, odir]
                                    ++ PD.includeDirs bi,

vs

      ghcOptCppIncludePath = odir : PD.includeDirs bi,

So all we need to do is change the latter to be the same as the former, so both include autogenModulesDir lbi.