remove $INCLUDE/$LIB hack for mingw

mingw is not msvc, and nobody in their right mind would expect it to
behave like it.

[ChangeLog][Important Behavior Changes][qmake] Qt configure and qmake
used with a MinGW spec will no longer emulate MSVC by picking up the
INCLUDE and LIB environment variables. Use the -I/-L configure options
to pass additional paths, as you would under Unix.

Change-Id: I533bb97de34d14dcbd45e0416283a79f44096c67
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>