Merge pull request #139 from GamePad64/master

Renamed "Windows.h" to "windows.h" for building spdlog on MinGW using case-sensitive file system.

include/spdlog/details/os.h

@@ -31,7 +31,7 @@
 # ifndef WIN32_LEAN_AND_MEAN
 #  define WIN32_LEAN_AND_MEAN
 # endif
-# include <Windows.h>
+# include <windows.h>
 
 #ifdef __MINGW32__
 #include <share.h>