Allow ares to be more easily built against external dependencies on
Windows outside of MSYS2 and using the MSVC ABI.
For example, dependencies can now acquired via vcpkg by installing the
relevant packages (pkgconf, gtk3, qt5, sdl2) and setting up the
environment appropriately before building.
```
CMD:
set PKG_CONFIG=C:\src\vcpkg\installed\x64-windows\tools\pkgconf\pkgconf.exe
set PKG_CONFIG_PATH=C:\src\vcpkg\installed\x64-windows\lib\pkgconfig
Bash:
export PKG_CONFIG=/c/src/vcpkg/installed/x64-windows/tools/pkgconf/pkgconf.exe
export PKG_CONFIG_PATH=/c/src/vcpkg/installed/x64-windows/lib/pkgconfig
```
Changes:
- hiro: fix building against gtk/qt with cl
- build: improve pkg-config detection and use with cl
- nall: support dynamic crt with clang-msvc/cl
0d131bf4d8