Replace the includes of Windows.h with windows.h (#204) (#235)

When cross-compiling with MinGW, the compiler is unable to find the
include of Windows.h because windows.h is provided instead.
Replacing the includes of Windows.h with windows.h solves the issue
and still works on Windows because the filesystem is case insensitive.

Author: ludovicdelfau

include/internal/common.hpp

@@ -13,7 +13,7 @@
 # ifndef WIN32_LEAN_AND_MEAN
 #  define WIN32_LEAN_AND_MEAN
 # endif
-# include <Windows.h>
+# include <windows.h>
 # undef max
 # undef min
 #elif defined(__linux__)

single_include/csv.hpp

@@ -1831,7 +1831,7 @@ using shared_ummap_sink = basic_shared_mmap_sink<unsigned char>;
 # ifndef WIN32_LEAN_AND_MEAN
 #  define WIN32_LEAN_AND_MEAN
 # endif
-# include <Windows.h>
+# include <windows.h>
 # undef max
 # undef min
 #elif defined(__linux__)

single_include_test/csv.hpp

@@ -1831,7 +1831,7 @@ using shared_ummap_sink = basic_shared_mmap_sink<unsigned char>;
 # ifndef WIN32_LEAN_AND_MEAN
 #  define WIN32_LEAN_AND_MEAN
 # endif
-# include <Windows.h>
+# include <windows.h>
 # undef max
 # undef min
 #elif defined(__linux__)