GIT_SILENT

It is actually not an available option; and thanks to how ExternalProject
is parsing the options, the `UPDATE_COMMAND ""` option that is often
conveniently placed immediately before that would cause CMake to always
rebuild the external project.

Removing them should stop most unnecessary rebuilds.

(Instead of MXE on Linux)

Qt is supposed to be built/installed separately because building Qt from
a complex cmake externals tree almost always fails somewhere.

Also missing are curl and libssh2 which are optional dependencies for
g'mic. poppler/freetype/fontconfig doesn't work on Linux yet, fontconfig
cannot find a locally installed freetype.

The internal dependencies graph isn't really filled in yet, because
trying to build all deps in one almost always fails anyway -- better
build the deps by hand or provide your own bat or sh script for that.

This now works on Linux; check 3rdparty/description for a small
howto. Next phase are adapting to OSX and Windows...