ANSI C

In computer programming, ANSI C (or ISO C) is a specification for the C language and an update to the original K&R version of C. Programs written the ANSI C language have access to a few extra features inspired by C++; the main difference between old C and ANSI C seems to be in declarations of function parameters. These days, ANSI C is routine and C programmers almost always use it.
 * The Portland Pattern Repository wiki has a page: Wiki:AnsiCee.
 * Wikipedia has one: ANSI C.

However, NetHack was a very old program dating from before ANSI C's first spec in 1989. Today's version can take advantage of certain ANSI C features, and code for this is in tradstdc.h. Is NetHack written in ANSI C? Yes and no, depending on what tradstdc.h decides to do.

The "void" type
In C, the "void" type indicates a function that does not return a value. The original C did not have a "void" type; programmers often declared functions to return "int" and ignore the value. (This is why the compiler never complains if you forget to return a value from a non-void function.)

It became common to  to cosmetically declare a void function. (The preprocessor would change every "void" to "int" and the C compiler would have no concept of void.) Later, many C compiler vendors started including the void keyword. C++ had a void keyword. So ANSI decided to put "void" in ANSI C.

If you find a void-free compiler to build NetHack with, then the procedure is to uncomment the  line at config.h so that tradstdc.h defines void.