alloca redeclaration warning on MinGW

[Hrvoje Niksic]

A tester tells me that the alloca declaration from the latest (CVS)
Autoconf manual, now installed in Wget, warns about symbol
redeclaration on MinGW:

#if HAVE_ALLOCA_H
# include <alloca.h>
#elif defined __GNUC__
# define alloca __builtin_alloca

The problem is that MinGW is using GNU C, but also #defines alloca to
__builtin_alloca in malloc.h, presumably to be compatible with MS VC.

One way to deal with this is to add "# undef alloca" before the
definition, but undeffing alloca defined by system libraries makes me
somewhat uneasy.  But then again, it would be only done for GCC, where
__builtin_alloca should always work, right?