xmalloca: small optimization

[Bruno Haible]

This is a small optimization: On platform without alloca(), the xmmalloca()
does not need to be defined, since nothing uses it.

2007-12-30  Bruno Haible  <address@hidden>

        * lib/xmalloca.c (xmmalloca): Don't define if HAVE_ALLOCA is not
        defined.

*** lib/xmalloca.c.orig 2007-12-30 16:44:24.000000000 +0100
--- lib/xmalloca.c      2007-12-30 16:34:13.000000000 +0100
***************
*** 22,27 ****
--- 22,29 ----
  
  #include "xalloc.h"
  
+ #if HAVE_ALLOCA
+ 
  void *
  xmmalloca (size_t n)
  {
***************
*** 32,34 ****
--- 34,38 ----
      xalloc_die ();
    return p;
  }
+ 
+ #endif