Resolves#10322
* Update MSYS2 download source. Resolve#10322
Proposal to resolve https://github.com/microsoft/vcpkg/issues/10322
* Use Sourceforge as MSYS2 download source (up to SF to choose mirror, fair to everyone and more reliable)
* Updated msys2 version to 2019-05-24 (Otherwise need to spend extra time on ```pacman -Syu```)
* Updated SHA512 accordingly
* Tested with building icu:x64-windows
* add official repo as backup
* [portfile functions] Override execute_process() to accept ALLOW_IN_DOWNLOAD_MODE option
* [vcpkg install] Set VCPKG_DOWNLOAD_MODE when using --only-downloads option
* [vcpkg_find_acquire_program] Allow in Download Mode
* Don't stop when build fails for a package
* Download sources for all packages in dependency graph
* Improve output messages
* Enable acquiring MSYS packages in download mode
* Documentation
* Update documentation
* execute_process() always fails on Download Mode
* Regenerate docs and fix formatting
* Run clang-format
* Use _execute_process on vcpkg_from_<source> helpers
* Fix calls to _execute_process() when not in Download Mode
* [portfile functions] Override execute_process() to accept ALLOW_IN_DOWNLOAD_MODE option
* [vcpkg install] Set VCPKG_DOWNLOAD_MODE when using --only-downloads option
* [vcpkg_find_acquire_program] Allow in Download Mode
* Don't stop when build fails for a package
* Download sources for all packages in dependency graph
* Improve output messages
* Enable acquiring MSYS packages in download mode
* Documentation
* Update documentation
* execute_process() always fails on Download Mode
* Regenerate docs and fix formatting
* Run clang-format
* Use _execute_process on vcpkg_from_<source> helpers
* Add doxygen to acquired programs
* Add packages option to msys function
* Add documentation for PACKAGES in vcpkg_require_msys
* [ffmpeg][x264] Update to use `PACKAGES` parameter. Tweak docs.
