Merge vestigial vsnprintf determination from zutil.h to gzguts.h.

This also moves some of the same from zconf.h to gzguts.h. A new
function, gzflags(), was created to pass the compilation flags
related to vsnprintf usage back to zlibCompileFlags() in zutil.c.
In the process, various compiler configuration files were updated
to include gzflags(), as well as the new gzgetc_() function added
when the gzgetc() macro was introduced in a previous patch.

zconf.h

@@ -61,8 +61,10 @@
 #  define gzdopen               z_gzdopen
 #  define gzeof                 z_gzeof
 #  define gzerror               z_gzerror
+#  define gzflags               z_gzflags
 #  define gzflush               z_gzflush
 #  define gzgetc                z_gzgetc
+#  define gzgetc_               z_gzgetc_
 #  define gzgets                z_gzgets
 #  define gzoffset              z_gzoffset
 #  define gzoffset64            z_gzoffset64
@@ -422,14 +424,6 @@ typedef uLong FAR uLongf;
 #endif
 #endif
 
-#if defined(__OS400__)
-#  define NO_vsnprintf
-#endif
-
-#if defined(__MVS__)
-#  define NO_vsnprintf
-#endif
-
 /* MVS linker does not support external names larger than 8 bytes */
 #if defined(__MVS__)
   #pragma map(deflateInit_,"DEIN")