Compile error when cross-compiling for Windows using MinGW-W64

Hey, thanks for making this library! I thought I'd update it today, but I now have this compile error when compiling for Windows:

```
loguru.cpp:(.text+0x2e97): undefined reference to `__imp__dupenv_s'
```

I include loguru.hpp like this:

```c++
#define LOGURU_WITH_STREAMS 1
#define LOGURU_THREADNAME_WIDTH 6
#include
```

I've modified `loguru.cpp` to include the above file rather than loguru.hpp, in order to get the correct settings

The Windows build environment can be reproduced here: https://gitlab.com/rubenwardy/docker-sfml-rvwp/blob/master/install_mingw.sh

Toolchain:

```cmake

set(CMAKE_SYSTEM_NAME Windows)

set(TOOLCHAIN_PREFIX x86_64-w64-mingw32)

# cross compilers to use for C and C++
set(CMAKE_C_COMPILER ${TOOLCHAIN_PREFIX}-gcc)
set(CMAKE_CXX_COMPILER ${TOOLCHAIN_PREFIX}-g++)
set(CMAKE_RC_COMPILER ${TOOLCHAIN_PREFIX}-windres)

SET( CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -static-libgcc -static-libstdc++" )

# target environment on the build host system
# set 1st to dir with the cross compiler's C/C++ headers/libs
set(CMAKE_FIND_ROOT_PATH /usr/${TOOLCHAIN_PREFIX} /usr/local/mingw64 ./extlibs)

set(LUA_INCLUDE_DIR /usr/local/mingw64/include/lua5.1/)
set(LUA_LIBRARY /usr/local/mingw64/lib/liblua5.1.a)
set(OPENAL_LIBRARY /usr/local/mingw64/lib/libopenal32.a)

# modify default behavior of FIND_XXX() commands to
# search for headers/libs in the target environment and
# search for programs in the build 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)
```