* xenial doesn't ship many dependencies, so make them all optional
since we don't guarantee that everything will work
* cmake/{5,6}: needs stdlib.h for EXIT_SUCCESS on GCC 5
* common/222: needs C++11, and GCC 5 doesn't understand `auto`
correctly unless we explicitly enable it.
* frameworks/1 boost: xenial doesn't ship boost_python3, so make it
properly optional
* frameworks/6 gettext: gettext can be installed without xgettext,
which doesn't cause the project to fail, but the installed files
list is different which causes the test to fail.
* frameworks/7 gnome: gobject-introspection can't be enabled because
the sanitizer unit test detects leaks in glib and fails
This fixes an issue with generated sources and object libraries, as
well as an issue on windows with the `link` linker and the vs backend.
The last issue is resolved by building the source files multiple times
to avoid extracting object files in meson.