A CODE block is C written by hand inside a 4GL program, and it reaches for
usleep as readily as for the standard functions the header chain already
provides. Without unistd.h those were implicitly declared, which was legal C89
and is an error in gcc 14 - so a program with such a block stopped compiling.
Not guarded on HAVE_UNISTD_H: the generated-code header chain does not pull in
a4gl_incl_config.h, so no HAVE_* symbol is defined by the time this is read.
Every platform this builds for - Linux, macOS, WSL2, MinGW-w64, FreeBSD -
provides the header, so it is included directly.
