Attempting to build with c header zpl-c/enet leads to an error: use of undeclared identifier 'CLOCK_MONOTONIC'

It’s generally not recommended to include the implementation of a library in a @cImport.
@cImport, which internally translates the C code to Zig code, is not working in many cases yet.
See Using a single-header C library from Zig - #2 by IntegratedQuantum for an explanation on how to deal with header-only libraries in zig.