...not only when building the libs themselves, but also when including their
header files from other code.  (Omission only becomes obvious with hidden
function type RTTI causing false positives from Clang -fsanitize=function.)  As
these external libs do not record the decision to enable visiblity in a config
header file that gets included, it appears easiest to hack that knowledge into
gbuild for now.  (Note that libodfgen internally uses librevenge.)

Change-Id: I6a3a722d561b8cbce6e5b1f27d7aa2d7602f3cdf