configure: don't export SYSTEM_VARIABLES to .qmake.vars

* linux-oe-g++ should handle this correctly and exporting LD as QMAKE_LINK is
  causing issues as we need g++ to be used as linker

Change-Id: I22e32aabd502df9f84cebb82f17bec4099363c8e
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
configure.prf: Allow to add extra arguments to make

* sometimes we would like to add -e or define some variable and respect it from both
  Makefiles used in configure tests and also Maakefiles to build the application

Change-Id: Ib29beb89cc9b179dddd40d3b212b7c200d29a469
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>