Eliminate msvc compiler warnings (fmtlib#931)

The sheer presence of 'std::gmtime' or 'std::localtime' gives rise to C4996 warnings in Microsofts compilers. Alas, the 'fallback(internal::null<>)'functions containing these library calls are never ADL-picked in the respective 'handle(internal::null<>)' selectors. Therefore hiding the fallbacks from msvc is perfectly fine.

include/fmt/time.h

@@ -51,7 +51,7 @@ inline std::tm localtime(std::time_t time) {
 
     bool fallback(int res) { return res == 0; }
 
-#ifndef _MSC_VER
+#if !FMT_MSC_VER
     bool fallback(internal::null<>) {
       using namespace fmt::internal;
       std::tm *tm = std::localtime(&time_);
@@ -90,7 +90,7 @@ inline std::tm gmtime(std::time_t time) {
 
     bool fallback(int res) { return res == 0; }
 
-#ifndef _MSC_VER
+#if !FMT_MSC_VER
     bool fallback(internal::null<>) {
       std::tm *tm = std::gmtime(&time_);
       if (tm) tm_ = *tm;