[llvm-3.5] Support GHCi and TH

[tmcdonell]

Enables llvm-general to be used out-of-the-box with GHCi and TemplateHaskell whenever possible. This requires the LLVM shared library to be available, but only if you use GHCi/TH; regular compiles work as usual. Since cabal dependencies can not specify flags, this also makes llvm-general easier to work with as a dependency.

Requires Cabal-1.22 or newer, otherwise keeps the old behaviour.

Fixes #84, #85.
Ping #197.

Current behaviour

• -fshared-llvm=True

  • executables are linked against the shared library, and required to compile llvm-general
  • GHCi and TH work

• -fshared-llvm=False (default)

  • executables are statically linked
  • GHCi and TH fail in strange and mysterious ways, e.g.:

ghc: libLLVMSupport.a: unhandled ELF relocation(RelA) type 42

or

    ghc:
    lookupSymbol failed in relocateSection (RELOC_GOT)
    /usr/local/homebrew/Cellar/llvm35/3.5.1/lib/llvm-3.5/lib/libLLVMSupport.a: unknown symbol `___dso_handle'

or maybe

    <no location info>:
        ghc: panic! (the 'impossible' happened)
      (GHC version 7.10.3 for x86_64-apple-darwin):
    	Dynamic linker not initialised

    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

New behaviour

• -fshared-llvm=True

  • same as above

• -fshared-llvm=False (default)

  • executables are statically linked
  • GHCi and TH work, if the shared library is available. If not, it still fails, but you get the more reasonable error message:

can't load .so/.DLL for: libLLVM-3.5.1.dylib (dlopen(libLLVM-3.5.1.dylib, 5): image not found)

[tmcdonell]

@bscarlet any comment on this?