Specify std version as gnu89

This lets clang compile lisp.c, which, unlike gcc, errors by default (instead of warning) on depreciated C89 constructs like implicit int unless c89/gnu89 is specifically selected. Tested with clang 18 and 19. Alternatively, any of the suggested fixes in #31 for gcc 14 would allow clang to compile this. However, this may still help clarify the intent of the code.