build, include alloca.h

Some systems (MacOSX) warned us about this.

Also see man 3 alloca:
 "Normally, gcc(1) translates calls to alloca() with inlined code. This
 is not done when either the -ansi, -std=c89, -std=c99, or the
 -fno-builtin option is  given (and the header <alloca.h> is not
 included).  But beware!  By default the glibc version of <stdlib.h>
 includes <alloca.h> and that contains the line:
   #define alloca(size)   __builtin_alloca (size)
 with messy consequences if one has a private version of this function."