configure: enable run-time-dependencies as build-time dependencies

configure: enable run-time-dependencies as build-time dependencies

With the recent addition of Sinhala keysyms, xkeyboard-config has a run-time
dependency on certain xproto/libX11 versions. The build will work anyway,
but using those layouts may result in unusable keyboards.

Enable these run-time dependencies as build-time dependencies so that those
building on their local machines are warned that the resulting build may not
work. Those building on build hosts (e.g. distros) can skip this check with
--disable-runtime-deps.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>