Avoid recursive gzgetc() macro call.

Recursive macro calls are normally caught by the preprocessor and
avoided. This commit avoids the possibility of a problem entirely.

zlib.h

@@ -1698,10 +1698,10 @@ ZEXTERN int ZEXPORT gzgetc_ OF((gzFile file));  /* backward compatibility */
 #ifdef Z_PREFIX_SET
 #  undef z_gzgetc
 #  define z_gzgetc(g) \
-          ((g)->have ? ((g)->have--, (g)->pos++, *((g)->next)++) : 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))
+          ((g)->have ? ((g)->have--, (g)->pos++, *((g)->next)++) : (gzgetc)(g))
 #endif
 
 /* provide 64-bit offset functions if _LARGEFILE64_SOURCE defined, and/or