make -Wl,-Bsymbolic and -Wl,-z,noexecstack optional

This fixes the configure step on OS X with Apple's provided clang (LLVM 3.4svn).
add missing include to cerrno

This fixes the build on OS X with Apple's clang (LLVM 3.4svn)
Merge pull request #4 from pigoz/fix-build-osx

Fix building on OS X