Fix problem with ./configure --prefix and gzgetc macro.

zlib.h

@@ -1651,8 +1651,14 @@ struct gzFile_s {
     z_off64_t pos;
 };
 ZEXTERN int ZEXPORT gzgetc_ OF((gzFile file));
-#define gzgetc(g) \
-    ((g)->have ? ((g)->have--, (g)->pos++, *((g)->next)++) : gzgetc_(g))
+#ifdef Z_PREFIX_SET
+#  undef z_gzgetc
+#  define z_gzgetc(g) \
+          ((g)->have ? ((g)->have--, (g)->pos++, *((g)->next)++) : gzgetc_(g))
+#else
+#  define gzgetc(g) \
+          ((g)->have ? ((g)->have--, (g)->pos++, *((g)->next)++) : gzgetc_(g))
+#endif
 
 /* provide 64-bit offset functions if _LARGEFILE64_SOURCE defined, and/or
  * change the regular functions to 64 bits if _FILE_OFFSET_BITS is 64 (if