Don't include the world with WIN32_LEAN_AND_MEAN (#503)

include/fmt/format.cc

@@ -22,6 +22,9 @@
 #endif
 
 #if FMT_USE_WINDOWS_H
+# if !defined(FMT_HEADER_ONLY) && !defined(WIN32_LEAN_AND_MEAN)
+#  define WIN32_LEAN_AND_MEAN
+# endif
 # if defined(NOMINMAX) || defined(FMT_WIN_MINMAX)
 #  include <windows.h>
 # else

include/fmt/posix.cc

@@ -19,6 +19,9 @@
 #ifndef _WIN32
 # include <unistd.h>
 #else
+# ifndef WIN32_LEAN_AND_MEAN
+#  define WIN32_LEAN_AND_MEAN
+# endif
 # include <windows.h>
 # include <io.h>