Squelch MSVC warning exporting subclasses of runtime_error

Based on pr fmtlib#1433 with changes to deal with issue fmtlib#1450

"When compiling {fmt} as a DLL, MSVC complains that we are exporting
classes that inherit from "std::runtime_error", which we are not
exporting.

In this case, it's not really a problem because that symbol is already
exported via the C++ stdlib. So we just add a pragma to silence the
warning."

Puts a pragma before the relevant class def instead of within it's
declaration for greater support among older msvc.

include/fmt/core.h

@@ -166,6 +166,12 @@
 #endif
 
 #if !defined(FMT_HEADER_ONLY) && defined(_WIN32)
+#  if FMT_MSC_VER
+#    define FMT_NO_W4275 __pragma(warning(suppress : 4275))
+#  else
+#    define FMT_NO_W4275
+#  endif
+#  define FMT_CLASS_API FMT_NO_W4275
 #  ifdef FMT_EXPORT
 #    define FMT_API __declspec(dllexport)
 #  elif defined(FMT_SHARED)

include/fmt/format.h

@@ -33,8 +33,6 @@
 #ifndef FMT_FORMAT_H_
 #define FMT_FORMAT_H_
 
-#include "core.h"
-
 #include <algorithm>
 #include <cerrno>
 #include <cmath>
@@ -43,6 +41,9 @@
 #include <memory>
 #include <stdexcept>
 
+#include "core.h"
+
+
 #ifdef __clang__
 #  define FMT_CLANG_VERSION (__clang_major__ * 100 + __clang_minor__)
 #else
@@ -688,6 +689,7 @@ using memory_buffer = basic_memory_buffer<char>;
 using wmemory_buffer = basic_memory_buffer<wchar_t>;
 
 /** A formatting error such as invalid format string. */
+FMT_CLASS_API
 class FMT_API format_error : public std::runtime_error {
  public:
   explicit format_error(const char* message) : std::runtime_error(message) {}
@@ -2699,6 +2701,7 @@ class arg_formatter : public internal::arg_formatter_base<Range> {
  An error returned by an operating system or a language runtime,
  for example a file opening error.
 */
+FMT_CLASS_API
 class FMT_API system_error : public std::runtime_error {
  private:
   void init(int err_code, string_view format_str, format_args args);