Correctly pass -platform to qtbase's configure script when CXX is an absolute path.

With the update to Qt 5.9.3, the configure script and qmake expect an mkspec
name, not an absolute path, which is why r458293 switched to using
${QMAKESPEC:T}.

However, the :T modifier breaks things when CXX is set to an absolute path
instead of just "c++", QMAKE_COMPILER is a shell string that will be evaluated
only after make invokes `configure' with CONFIGURE_ARGS. In other words, we end
up turning something like

$$(ccver="$$(/usr/bin/c++ --version)"; case "$$ccver" in *clang*) echo clang ;; *) echo g++ ;; esac)

into

/c++ --version)"; case "$$ccver" in *clang*) echo clang ;; *) echo g++ ;; esac)

which is obviously invalid.

We now just avoid being too smart and set a separate variable called
QMAKESPECNAME, which contains only the mkspec name and that we use both when
setting CONFIGURE_ARGS as well as to create QMAKESPEC.

PR: 224971