qextserialport: use TARGET_MAKE_ENV when running make and qmake

This patch prepends TARGET_MAKE_ENV to $(MAKE) and $(QT_QMAKE) command
lines, to make sure PATH is set correctly, and to line up with the qt5
packages that do this consistently.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>