ports-mgmt/pkg: Respect liblzma.pc if available.

Note that this is meant to be a temporary hack and will be reverted once the
freebsd/pkg have solved this in the build infrastructure.

PR: 200142
MFH: 2020Q2
Approved by: portmgr (bapt over IRC)
Differential Revision: https://reviews.freebsd.org/D24972