Define "__GHCIDE__" on CPP

[serras]

This patch comes from my experience in using ghcide in combination with quasi-quoters which read from files. If you have something like:

$(doSomething "from.file")

then build tools would take the relative path to start with the source folder of the corresponding project, but ghcide takes it from the root of the project. Until a better solution is found, my proposal is to have this additional define in CPP, so we can do:

#if __GHCIDE__
$(doSomething "src/from.file")
#else
$(doSomething "from.file")
#endif

[cocreature]

Do you have an idea of how hard it would be to fix this properly? I would very much like to avoid adding more GHCIDE specific hacks and rather invest into making sure that we agree with GHC.

[serras]

My gut feeling is that once we have multi-cradle support from hie-bios, we should be able to configure it there.

[ndmitchell]

I think it's pretty reasonable to have a macro for ghcide. It's standard practice for most tools - eg HLint defines one. Would be great if no one ever needed it, but that's probably unrealistic.

[cocreature]

HLint doesn’t try to be a Haskell compiler so I don’t think this is a fair comparison. That said, I tend to agree that this macro is reasonable so let’s merge this for now.