[BUG] assert() gets redefined by standard assert.h

[borup3]

Describe the bug
Zen C outputs a #define assert(cond, ...) to the C file but this easily gets redefined by included C libraries, like assert.h on macOS which is often included in other standard libraries or open-source projects.

To Reproduce
Steps to reproduce the behavior:

1. Include https://github.com/septag/dmon/blob/master/dmon.h
2. Build Zen C program with an assert call in it.
3. See C errors.

Expected behavior
Zen C defines its own assert that doesn't clash with the standard assert and rewrites all assert() calls in Zen C to this, e.g. __zenc_assert().

Environment (please complete the following information):

• OS: macOS