orcus upgrade requires "make sc.clean" (except possibly on windows)

[Michael Stahl]

the orcus upgrade on master today requires a "make sc.clean" for
incremental builds.

the reason is that a whole bunch of tests were using "orcus" and
"orcus-parser" externals so were linked against those libraries, but
their cxx files didn't actually include any orcus header, so the cxx
files were not rebuilt, and the test libraries were not re-linked
because none of the gbuild-built libraries it depends on changed their
exported symbols so the optimization that doesn't re-link libraries
unnecessarily kicks in, and the test libraries drag the previous orcus
libraries into the cppunittester process.

except on Windows, where orcus library itself is built with gbuild, so
the rebuild of the orcus library with different exported symbols causes
all the tests to relink.

i've removed the unnecessary orcus linking in commit
07d4c443e21cd22545d4516515c50b1c8dd39a9b  so let's hope that the next
upgrade will go better :)