NXC Version 1.2.1 r5
The #include command works as in Standard C, with the caveat that the filename must be enclosed in double quotes.
There is no notion of a system include path, so enclosing a filename in angle brackets is forbidden.
NXC programs can begin with #include "NXCDefs.h" but they don't need to. This standard header file includes many important constants and macros, which form the core NXC API. NXC no longer require that you manually include the NXCDefs.h header file. Unless you specifically tell the compiler to ignore the standard system files, this header file is included automatically.