fix building on BSD systems which often do not have alloca.h

--- sed-4.1.5/lib/regex_internal.h
+++ sed-4.1.5/lib/regex_internal.h
@@ -410,7 +410,9 @@ static unsigned int re_string_context_at
 #define re_string_skip_bytes(pstr,idx) ((pstr)->cur_idx += (idx))
 #define re_string_set_index(pstr,idx) ((pstr)->cur_idx = (idx))
 
+#ifdef HAVE_ALLOCA_H
 #include <alloca.h>
+#endif
 
 #ifndef _LIBC
 # if HAVE_ALLOCA