[Doxygen] Ensure `libs.C` is ACLiC'ed before `modifyClassWebpage.sh` runs

`modifyClassWebpages.sh` might spawn many `modifyClassWebpage.sh` processes
via `xargs -P`. In turn, `modifyClassWebpage.sh` depends on `libs.C`
which is ACLiC'ed if to generate `libs_C.so` where required.

However, if `libs_C.so` does not exist, multiple processes might race to
create it. This commit ensures that the macro is ACLiC'ed before
`modifyClassWebpage.sh` runs.