[3.0.x backport] 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.

Fixes #26888

Cherry-picked from commit 346b2fb9.

Signed-off-by: Alexandre Janniaux ajanni@videolabs.io

changed milestone to %3.0.x maintenance

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added 5 commits

• 846e8588...41d862cc - 4 commits from branch videolan:3.0.x
• 094970ab - contrib: main.mak: force LIBDIR to lib/

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enabled an automatic merge when the pipeline for 094970ab succeeds

merged

changed milestone to %3.0.18