Fix printf format warnings on mingw clang

[guusw]

Hey,
When compiling with mingw bundled clang, it will complain that the gnu_printf format argument is not supported.
using the other macro with printf however works fine.

./imgui.h:608:118: warning: 'format' attribute argument not supported: gnu_printf [-Wignored-attributes]
    IMGUI_API bool          TreeNodeExV(const char* str_id, ImGuiTreeNodeFlags flags, const char* fmt, va_list args) IM_FMTLIST(3);
                                                                                                                     ^
./imgui.h:98:52: note: expanded from macro 'IM_FMTLIST'
#define IM_FMTLIST(FMT)             __attribute__((format(gnu_printf, FMT, 0)))
                                                   ^

I changed the conditions so it will use format(printf, ...) on mingw clang, but still use format(gnu_printf,...) for mingw gcc

Tested with mingw clang/gcc and also native windows clang. All report no errors with this fix.

>g++ --version
g++.exe (Rev5, Built by MSYS2 project) 10.3.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
>clang++ --version
clang version 12.0.1
Target: x86_64-w64-windows-gnu
Thread model: posix
InstalledDir: C:/msys64/mingw64/bin
>/a/LLVM/bin/clang++ --version              
clang version 12.0.1
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: A:\LLVM\bin

[ocornut]

Merged, thank you!