Log Message:
motif: pull libXt as a tool dependency only if using modular xorg *and*
cross-compiling.

using TOOL_DEPENDS for libXt bypasses normal builtin checks, so the
pkgsrc version is pulled in, and libXt is marked unavailable when using
native x.

evidence so far indicates libxt is only needed as a tool dependency
when cross-compiling (the distinction between normal and tool depends
primarily being relevent during cross-builds anyway), but this
also confused me because libXt installs nothing tool-like? Is motif's
build system broken (it's likely!)?