Replace --no-{rtti,exceptions} by -fno-{rtti,exceptions}

This improves compatibility with clang. Moreover, to the best
of my knowledge, the --no-* variants are undocumented to start with.
At least the GCC manuals for all versions from 2.95.3 till 4.7.2
only list the -fno-* variants, as far as I could tell.
Silence some warnings about statements without effect
Fix main() prototype of libparse
Do not specify default parameter in an extern prototype
Fix spelling: "MacOS X" -> "Mac OS X"

The correct official spelling is indeed "Mac OS X". This is in
contrast to old macintosh operating systems which were called "MacOS"
as in "MacOS 7".
Fix typos and spelling in README
Include stddef.h before gmp.h

This is required for clang because stddef.h is subtly different depending on whether
compiling in C or C++ mode.
Merge pull request #247 from fingolfin/clang-osx-patches

Patches for OS X and clang, plus some other cleanup