Teach pathfix.mk about LIB_DESTINATION

Some cmake projects use LIB_DESTINATION/pkgconfig to install the pc-files.

For example:

• deskutils/libstreamanalyzer
• sysutils/polkit-qt{,5}
• x11-toolkits/attica

By adding this, we can convert these ports to use pathfix instead of an
uncessessary patch-foo or confusing sed call.

Reviewed by: mat
Approved by: portmgr (mat), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D8141