[qt5-build.eclass] Export toolchain variables only when needed.

The variables exported by tc-export are used only by the configure script.
[qt5-build.eclass] Minor refactoring.
[qt5-build.eclass] Fix comment.
[qt5-build.eclass] Remove useless sed.

The intent of these two expressions was to give precedence to
the user's CXXFLAGS/LDFLAGS over those passed by the configure
on the command line. However the user's flags are stored in
.qmake.cache, which is read before processing command line
variables anyway, thus there's no point in changing the order.
Some other solution is needed.