chore(cmake): suppress clang warning

mismatched-tags warning: it's a problem due to gcc-4.8.1's stdlibc++,
which is already fixed in gcc-4.8.2 but Ubuntu-12.04 doesn't support it
yet.

null-conversion warning: The generated code from flex has lines which
cause the warning. We disable this warning only in the parser
subdirectory.
fix(realpaver): fix code to remove clang++ warnings
fix(opensmt): fix to remove clang++ warnings