Correct compile errors on Mac Clang 5.0.

I heeded your advice and have worked up these 3 commits. These only correct the compiler errors on Mac and do not address the linker errors. I'll address the linker issues in separate pull request after we have worked out these issues.

I added the off_t asserts and I've also included links to the C99 standard in the b3be3e0 commit message to clarify the 'j' length modifier and the 'intmax_t' type.

Let me know what you think.