deb: put mythplugins in section metapackages
deb: pass LDFLAGS to qmake during configure

Currently only configured to do it on Darwin
as of mythtv git 5dccbd5f6b3019a67ecc69213ee405bba4b30994

Needed to make hardening work properly with debhelper 9.
DH9 will automatically set the env for LDFLAGS, CFLAGS etc
See for more info: https://wiki.debian.org/Hardening