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

a81a7dc fixes a typo in src/full/Agda/Compiler/JS/Syntax.hs that breaks building completely if Clang's cpp is used.

943ea3c changes Agda.cabal and mk/config.mk.in to use cpphs by default for GHC's CPP extension. This should fix build failures in preprocessing on all systems whose C preprocessor behaves non-traditionally by default. (It also reformats README.md and drops the outdated Windows Emacs advice in favor to a link to the binaries built by Aaron Stump.)
Since the CPP extension is enabled, added should preprocessing ever be necessary on executable-only sources. But since Main.hs is just a stub, it isn't necessary at present; no problem with removing.