driconf: Add proper dependency for compiling .mo files from .po files.

driconf: Add proper dependency for compiling .mo files from .po files.

Previously this was happening unconditionally, leading to some excessive
rebuilding/relinking during builds.

Note that the .po files are not automatically updated due to changes to the
t_options.h file. Instead, translators should continue to use "make po"
manually. This is because after new strings are merged into the existing .po
file, manual work is still required by translators to ensure that the
translations are correct.