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.

gzguts.h

@@ -31,9 +31,48 @@
 #  define NO_GZCOMPRESS
 #endif
 
-#ifdef _MSC_VER
-#  include <io.h>
-#  define vsnprintf _vsnprintf
+#if defined(STDC99) || (defined(__TURBOC__) && __TURBOC__ >= 0x550)
+#  ifndef HAVE_VSNPRINTF
+#    define HAVE_VSNPRINTF
+#  endif
+#endif
+
+#if defined(__CYGWIN__)
+#  ifndef HAVE_VSNPRINTF
+#    define HAVE_VSNPRINTF
+#  endif
+#endif
+
+#ifndef HAVE_VSNPRINTF
+#  ifdef MSDOS
+/* vsnprintf may exist on some MS-DOS compilers (DJGPP?),
+ but for now we just assume it doesn't. */
+#    define NO_vsnprintf
+#  endif
+#  ifdef __TURBOC__
+#    define NO_vsnprintf
+#  endif
+#  ifdef WIN32
+/* In Win32, vsnprintf is available as the "non-ANSI" _vsnprintf. */
+#    if !defined(vsnprintf) && !defined(NO_vsnprintf)
+#      if !defined(_MSC_VER) || ( defined(_MSC_VER) && _MSC_VER < 1500 )
+#         include <io.h>
+#         define vsnprintf _vsnprintf
+#      endif
+#    endif
+#  endif
+#  ifdef __SASC
+#    define NO_vsnprintf
+#  endif
+#  ifdef VMS
+#    define NO_vsnprintf
+#  endif
+#  ifdef __OS400__
+#    define NO_vsnprintf
+#  endif
+#  ifdef __MVS__
+#    define NO_vsnprintf
+#  endif
 #endif
 
 #ifndef local