Add ability to compile with clang without libc++.

On linux systems that do not provide libc++ by default, but that _do_ provide a build of clang/clang++, it's useful to be able to compile the project without using libc++ directly.

To do so, I've added a CPP-NETLIB_DISABLE_LIBCXX option to the main CMakeLists.txt---this option defaults to OFF (so as to not change the default behavior when compiling with clang) but now allows us non-OSX people to use -DCPP-NETLIB_DISABLE_LIBCXX=ON when generating build files to still use clang for compilation but build against the system default c++ standard library.

I'm also noticing that doing so was a bit of shotgun surgery on all of the associated CMakeLists.txt---is there some way we can unify the detection of CFLAGS so this isn't duplicated everywhere? I didn't investigate this too much because of my unfamiliarity with CMake.