Portability Fixes

[donn]

These are a small number of changes that allow cvc to compile on more platforms.

• malloc.h is not standard and was replaced with cstdlib
• stdint-gcc.h has been superseded by cstdint
• INTLLIBS added explicitly (not found on macOS otherwise)

This does not enable compilation using Clang: obstack just wouldn't have it. But it allows it to compile on macOS reasonably

[d-m-bailey]

@donn Thanks! I'll check the changes and probably merge them next week.