There are unix systems (like FreeBSD) that provide an alloca function
in their libc but don't have a header file alloca.h . alloca is
declared in some other header file (stdlib.h, for example). The
autoconf macro AC_FUNC_ALLOCA handles this situation by defining
HAVE_ALLOCA but not HAVE_ALLOCA_H.
On these systems, alloca is currently redefined in src/nconfig.h
because HAVE_ALLOCA is not checked there.
The attached patch modifies src/nconfig.h so that alloca is only
defined there if alloca.h is absent and alloca is not found by the