replace mechanism to override variables from the mkspecs

it is sometimes desirable to override values from the mkspec without
modifying (or wrapping) the spec itself. linux distributors do this on a
regular basis.

so far, we'd pick up CFLAGS, etc. from the environment, in a somewhat
autoconf-like fashion. however, over time, this approach proved
problematic: the concept doesn't mix particularly well with mkspecs to
start with, is unexpected (and therefore causes frustration), and
doesn't mix well with cross-building (at least the way it was realized).
ironically, it was implemented this way (quite a while ago) upon my
explicit request ...

the new mechanism uses explicit variable manipulations on the configure
command line, just like qmake itself understands. as it happens, this is
again quite similar to autoconf-generated configure scripts. however,
this time around we don't pretend to be actually autoconf-like, so we
also don't try to map any variable names (some of which have different
semantics anyway).

this commit also eliminates the last use of the QMakeVar() function,
so delete it and the underlying infrastructure.

Task-number: QTBUG-32530
Task-number: QTBUG-42962
Change-Id: Id31a6b80e1add08ca21f5b178614bda530d12374
Reviewed-by: Lars Knoll <lars.knoll@qt.io>