Add warnings when compiling with assembler code.

There have been many reports of bugs in the assembler codes
intended to speed up deflate and inflate. They are third-party
contributions in contrib, and so are not supported by the zlib
maintainers.
diff --git a/deflate.c b/deflate.c
index 7817530..9161f29 100644
--- a/deflate.c
+++ b/deflate.c
@@ -88,6 +88,7 @@
 local void flush_pending  OF((z_streamp strm));
 local unsigned read_buf   OF((z_streamp strm, Bytef *buf, unsigned size));
 #ifdef ASMV
+#  pragma message("Assembler code may have bugs -- use at your own risk")
       void match_init OF((void)); /* asm code initialization */
       uInt longest_match  OF((deflate_state *s, IPos cur_match));
 #else
diff --git a/inffast.c b/inffast.c
index a15876d..29eb7d8 100644
--- a/inffast.c
+++ b/inffast.c
@@ -8,7 +8,9 @@
 #include "inflate.h"
 #include "inffast.h"
 
-#ifndef ASMINF
+#ifdef ASMINF
+#  pragma message("Assembler code may have bugs -- use at your own risk")
+#else
 
 /*
    Decode literal, length, and distance codes and write out the resulting