incremental rebuilds after icu bump

[Caolán McNamara]

bumping icu to the latest version has revealed a bit of a gotcha with
the delivering of headers from some of those third-party libraries into
the solver

the icu/prj/d.lst delivers headers right out of the build tree into the
solver. deliver carefully retains the initial timestamps of those files,
and they were untarred to retain their original timestamps

i.e. the non-generated-during-the-build icu headers are timestamped Apr
05 2012 or some such, so if you can't incrementally rebuild because some
stuff isn't getting correctly rebuilt (or some
generated-header-in-old-icu isn't getting replaced by a
nongenerated-bundled-header-in-new-icu) try a manual...

rm -rf solver/*/inc/external/unicode/ solver/*/inc/external/layout/

C.