Fix build break on *BSD

Linux uses alloca.h; many other operating systems define alloca(3) in
stdlib.h.

Signed-off-by: Garrett Cooper <yanegomi@gmail.com>

include/jemalloc/internal/jemalloc_internal.h.in

@@ -359,7 +359,11 @@ static const bool config_ivsalloc =
 #    include <malloc.h>
 #    define alloca _alloca
 #  else
-#    include <alloca.h>
+#    ifdef JEMALLOC_HAS_ALLOCA_H
+#      include <alloca.h>
+#    else
+#      include <stdlib.h>
+#    endif
 #  endif
 #  define VARIABLE_ARRAY(type, name, count) \
 	type *name = alloca(sizeof(type) * count)