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.

deflate.c

@@ -88,6 +88,7 @@ local void putShortMSB    OF((deflate_state *s, uInt b));
 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

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