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.

zutil.c

@@ -85,31 +85,7 @@ uLong ZEXPORT zlibCompileFlags()
 #ifdef FASTEST
     flags += 1L << 21;
 #endif
-#if defined(STDC) || defined(Z_HAVE_STDARG_H)
-#  ifdef NO_vsnprintf
-        flags += 1L << 25;
-#    ifdef HAS_vsprintf_void
-        flags += 1L << 26;
-#    endif
-#  else
-#    ifdef HAS_vsnprintf_void
-        flags += 1L << 26;
-#    endif
-#  endif
-#else
-        flags += 1L << 24;
-#  ifdef NO_snprintf
-        flags += 1L << 25;
-#    ifdef HAS_sprintf_void
-        flags += 1L << 26;
-#    endif
-#  else
-#    ifdef HAS_snprintf_void
-        flags += 1L << 26;
-#    endif
-#  endif
-#endif
-    return flags;
+    return flags + gzflags();
 }
 
 #ifdef DEBUG