x11-toolkits/pango: decouple MANPAGES from DOCS

MANPAGES_IMPLIES was set to not duplicate BUILD_DEPENDS lines, but
textproc/py-gi-docgen is not used for man page generation. Additionally,
note that DOCS generation with the current textproc/py-gi-docgen
may not be deterministic in certain conditions.

While here, use DISTVERSION.