Allow to cross-compile Windows build under Linux using MinGW.

It works fine on Ubuntu. You only need to use:
cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/Toolchain-mingw.cmake

cmake/Toolchain-mingw.cmake

@@ -0,0 +1,17 @@
+# the name of the target operating system
+SET(CMAKE_SYSTEM_NAME Windows)
+
+# which compilers to use for C and C++
+SET(CMAKE_C_COMPILER i686-w64-mingw32-gcc-posix)
+SET(CMAKE_CXX_COMPILER i686-w64-mingw32-g++-posix)
+SET(CMAKE_RC_COMPILER i686-w64-mingw32-windres)
+
+# here is the target environment located
+SET(CMAKE_FIND_ROOT_PATH /usr/i686-w64-mingw32 ${PROJECT_SOURCE_DIR}/dependencies)
+
+# adjust the default behaviour of the FIND_XXX() commands:
+# search headers and libraries in the target environment, search
+# programs in the host environment
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

lib/enet/CMakeLists.txt

@@ -22,7 +22,7 @@ add_library(enet STATIC
 )
 
 if(MINGW)
-  target_link_libraries(enet wsock32 ws2_32 Winmm)
+  target_link_libraries(enet wsock32 ws2_32 winmm)
 endif()
 
 #if(WIN32)

src/graphics/irr_driver.cpp

@@ -82,7 +82,7 @@
 using namespace irr;
 
 #ifdef WIN32
-#include <Windows.h>
+#include <windows.h>
 #endif
 #if defined(__linux__) && !defined(ANDROID)
 #include <X11/Xlib.h>

src/io/file_manager.cpp

@@ -38,11 +38,11 @@
 #include <iostream>
 #include <string>
 
-namespace irr {
-    namespace io
-    {
-        IFileSystem* createFileSystem();
-    }
+namespace irr {
+    namespace io
+    {
+        IFileSystem* createFileSystem();
+    }
 }
 
 // For mkdir
@@ -53,7 +53,7 @@ namespace irr {
 #  include <unistd.h>
 #else
 #  include <direct.h>
-#  include <Windows.h>
+#  include <windows.h>
 #  include <stdio.h>
 #  if !defined(__CYGWIN__ ) && !defined(__MINGW32__)
      /*Needed by the remove directory function */

src/network/stk_host.cpp

@@ -26,7 +26,7 @@
 
 #include <string.h>
 #if defined(WIN32)
-#  include "Ws2tcpip.h"
+#  include "ws2tcpip.h"
 #  define inet_ntop InetNtop
 #else
 #  include <arpa/inet.h>