Use external interpreter when compiling with profiling

[mrkkrp]

Currently (as of #332) to compile modules that use TH with profiling we also compile them without profiling because RTS that GHC is linked with is without profiling and so it needs object files without profiling to run TH. This is not very nice and probably less efficient than using the newer -fexternal-interpreter that was introduced in GHC 8.0 to solve exactly that problem.

See also:

• https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html?highlight=#ghc-flag--fexternal-interpreter
• https://ghc.haskell.org/trac/ghc/wiki/RemoteGHCi

[mrkkrp]

I tried to mess with this one today and discovered the following:

• To register a package dynamic library has to be present, otherwise ghc-pkg complains.
• We still cannot compile dynamic library with profiling because GHC doesn't come with dynamic object files with profiling enabled. -fexternal-interpreter doesn't seem to help in that case.

[mboes]

Could we use ghc-pkg --force here? It looks safe to me in this case, but you'd have to think carefully about whether that's really the case.

[mrkkrp]

An update: while working on "arbitrary file layouts" I noticed that -fexternal-interpreter probably should work if we don't set extensions to anything non standard. GHC should pick whatever it wants, i.e. just plain .o then it should start working.

[guibou]

I wonder if this issue can be closed, apparently rules_haskell is already using -fexternal-interpreter, see the discussion in #438 for details.