Fix mingw build

[cloudwu]

I have tried to build RmiUI with mingw/gcc today, and I found

1. The samples need link shlwapi, but https://github.com/mikke89/RmlUi/blob/master/Samples/shell/src/win32/ShellWin32.cpp#L37 only works for msvc.
2. mingw/gcc has already defined the macro NOMINMAX.
3. mingw/gcc has not defined the macro WIN32. (Only used in https://github.com/mikke89/RmlUi/blob/master/Samples/basic/treeview/src/FileSystem.cpp , and I guess using RMLUI_PLATFORM_WIN32 instead would be better ? )

This PR may fixes these issues.

[mikke89]

Thank you for the PR!

Can I ask for these changes:

• I'd prefer it if we replaced the WIN32 macro with RMLUI_PLATFORM_WIN32 as you suggest.
• With the changed CMake configuration we should be able to remove #pragma lib.

We should probably update the Appveyor configuration to also test mingw, but we can do that later.

[rokups]

add_definitions(-DWIN32)

• Never use add_definitions(), always use target_compile_definitions(). Seems like CMakeLists.txt could use a cleanup.
• MinGW defines _WIN32, same as original windows SDK. _WIN32 should be used in some header to define RMLUI_PLATFORM_WIN32 if we want a nicer-looking define.

target_link_libraries(${NAME} shlwapi)

Could be improved by changing it to target_link_libraries(${NAME} PUBLIC shlwapi) to clearly state that targets linking to our library should also link to shlwapi.

[mikke89]

• Never use add_definitions(), always use target_compile_definitions(). Seems like CMakeLists.txt could use a cleanup.

Yeah, we have a lot of legacy baggage here. If anyone is up to the task... ;)

• MinGW defines _WIN32, same as original windows SDK. _WIN32 should be used in some header to define RMLUI_PLATFORM_WIN32 if we want a nicer-looking define.

We already have this define, just need to make use of it in this case.

target_link_libraries(${NAME} shlwapi)

Could be improved by changing it to target_link_libraries(${NAME} PUBLIC shlwapi) to clearly state that targets linking to our library should also link to shlwapi.

To be clear, only the sample 'shell' uses this library. It should be sufficient to declare only on the shell target (with the PUBLIC clarifier) which would be preferable.

[cloudwu]

Done.

[mikke89]

Great one, thank you!