* Add support for building with MinGW
Tested with MSYS2 MinGW 8.3.0, gcc-mcf.lhmouse MinGW 9.2.1,
and StephanTLavavej/mingw-distro!
* Add MinGW toolchain
From your MinGW configured shell you could just use vcpkg to
configure packages.
An x64-mingw triplet would look like:
```
set(VCPKG_TARGET_ARCHITECTURE x64)
set(VCPKG_CRT_LINKAGE dynamic)
set(VCPKG_LIBRARY_LINKAGE static)
set(VCPKG_ENV_PASSTHROUGH PATH)
set(VCPKG_CMAKE_SYSTEM_NAME MinGW)
```
* Add MinGW community tripplets
x64 tested with https://github.com/StephanTLavavej/mingw-distro
x86, arm64, arm tested with https://github.com/mstorsjo/llvm-mingw
* [vcpkg] add missing implib definitions, fix shared lib extension on mac
(cherry picked from commit 2cddef9541)
* Better find_library suffixes and prefixes.
* moved additional suffix into a comment and reverted back to what is expected on windows
* updated documentation for add variables.
* Introduce an easier way to identify target systems...
* fix missing ()
* move the include around.
also added vcpkg_common_functions to be automatically included
* remove warning message
* added remaining VCPKG_CMAKE_SYSTEM_NAMEs found only in vcpkg_configure_cmake to load a toolchain.
* remove warning again from messed up merge
