Previously building with `cmake -G Ninja` would fail with this error:
ninja: error: 'third-party/event-c51b159cff/lib/libevent.a', needed by
'libtransmission/transmission-test-watchdir_generic', missing and no
known rule to make it
This was due to missing BUILD_BYPRODUCTS in the calls to
ExternalProject_Add(), as discussed here:
<https://cmake.org/pipermail/cmake/2015-April/060234.html>
Standard Windows programs (e.g. Explorer) tolerate this, but some other
programs (e.g. Firefox) are more strict in how they retrieve version
information and may fail if language IDs in StringFileInfo and VarFileInfo
don't match.
