contrib: main.mak: force LIBDIR to lib/

CMAKE_INSTALL_LIBDIR can be lib, lib64 or even lib/ on Debian, but we already split the contribs by multiarch-tuple / installation directory at the prefix level for contribs to avoid mixups.

Preventing installation in lib64/ ensure every pc files from contribs/ are in the PKG_CONFIG_PATH when configuring.

Needs testing first.