Remove LIBDIR from build env variable

This is leaking LIBDIR information (which is defined in base share/mk/*) to
the build environement with a value which is not really suitable for ports
(/usr/lib) which may have an impact on some build system forcing the maintainer
to override the value.

Revealed during the update of liblz4

PR: 228059
Submitted by: bapt
Exp-run by: antoine