Recent versions of Clang's CPP choke on '#-}' if in the correct position to parse as a preprocessing directive. 

This fixes building on OSX with Xcode.

x86_64-apple-darwin14: clang-3.5, cabal 1.16.0.2, ghc 7.6.3
Switched to using cpphs for preprocessing Haskell files.
Builds under OSX 10.10 with Xcode; agda --test succeeds.

Agda.cabal:
- Added cpphs as a build-tool
- Changed ghc-options to use cpphs --cpp as the Haskell source preprocessor. (cpphs without --cpp doesn't like the other build options.)

mk/config.mk.in:
- Added `-pgmPcpphs -optP--cpp` to GHC_FLAGS

README.md:
- Reformatted and swapped installation and prerequisite order
- Added cpphs to dependencies; added a note on how to use to compile dependencies installed by cabal
- Removed extremely outdated Windows Emacs advice; replaced with a link to Aaron Stump's precompiled version
Merge pull request #3 from coruus/fix-build-osx

Fix building on systems with non-traditional CPPs (including OSX)