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 (It also reformats README.md and drops the outdated Windows Emacs advice in favor to a link to the binaries built by Aaron Stump.)
Maybe my question wasn't clear. I understand why you added the options in the *library* section, but I guess the options aren't needed in the *executable* section.