[mingw] add VCPKG cmake_system_name

[MVoz]

Supports: !mingw or Supports: (mingw|windows) ...

vcpkg_fail_port_install(MESSAGE "Only ... is supported by ${PORT}" ON_TARGET MinGW)

logic

VCPKG_CMAKE_SYSTEM_NAME MinGW

vcpkg/triplets/community/x64-mingw.cmake

Line 6 in ffa7fd2

set(VCPKG_CMAKE_SYSTEM_NAME MinGW)

---

example

VCPKG_CMAKE_SYSTEM_NAME WindowsStore

vcpkg/triplets/community/arm64-uwp.cmake

Line 5 in ffa7fd2

set(VCPKG_CMAKE_SYSTEM_NAME WindowsStore)

VCPKG_CMAKE_SYSTEM_NAME Emscripten

vcpkg/triplets/community/wasm32-emscripten.cmake

Line 14 in ffa7fd2

set(VCPKG_CMAKE_SYSTEM_NAME Emscripten)

---

why is there no compiler definition? as in the example

https://gitlab.kitware.com/cmake/community/-/wikis/doc/cmake/cross_compiling/Mingw

https://gitlab.kitware.com/cmake/community/uploads/265a703419aeaa26bef8044b92911e37/Toolchain-cross-mingw32-linux.cmake

# for 32 or 64 bits mingw-w64
# see http://mingw-w64.sourceforge.net/
set(COMPILER_PREFIX "i686-w64-mingw32")
#set(COMPILER_PREFIX "x86_64-w64-mingw32"

# which compilers to use for C and C++
find_program(CMAKE_RC_COMPILER NAMES ${COMPILER_PREFIX}-windres)
#SET(CMAKE_RC_COMPILER ${COMPILER_PREFIX}-windres)
find_program(CMAKE_C_COMPILER NAMES ${COMPILER_PREFIX}-gcc)
#SET(CMAKE_C_COMPILER ${COMPILER_PREFIX}-gcc)
find_program(CMAKE_CXX_COMPILER NAMES ${COMPILER_PREFIX}-g++)
#SET(CMAKE_CXX_COMPILER ${COMPILER_PREFIX}-g++)


# here is the target environment located
SET(USER_ROOT_PATH /home/erk/erk-win32-dev)
SET(CMAKE_FIND_ROOT_PATH  /usr/${COMPILER_PREFIX} ${USER_ROOT_PATH})

# 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)

[c72578]

true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "mingw")
->
true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "MinGW")

[MVoz]

ok

[PhoebeHui]

LGTM, thanks for the PR!