In computer programming, ANSI C (or ISO C, or C89) 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.
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
#define void int 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.
Function declarations in ANSI C
Note for the ambitious newbie
In case you consider creating a project of your own, be it a Rogue-like game or some other application, consider using a more modern language than C. Nowadays there are lots of platform-independent, high-level alternatives like Java, Python or Perl just to mention a few. Such an application would be far easier to debug and maintain than its counterpart written in C. If you feel an urge to squeeze a bit more power out of the machine for your advanced ANSI-graphics and its pixelshading algorithms, at least consider using C++. If you, despite every sane thought, decide that C is the language you want to use, use the latest version of C, C99. The latest version contains many corrections and improvements and will cause you less trouble.
If you choose to develop your code in C or C++, and are using something like gcc to compile, use the options "-Wall -ansi -pedantic". You can also use "-std=c99" for the C99 version of C.
Vanilla NetHack continues to use C because of inertia (a 150,000+ line program is non-trivial to translate) and because of its stated goal: to get the game working on as many different types of hardware and under as many different operating systems as is practical.