rather than reproducing vcvarsall.bat's functionality as hard-wired code
in the nmake generator, just invoke the actual script from
toolchain.prf. this is much easier, more future proof, and - critically
- makes the detected variables available to configure's new library &
header search facilities.
[ChangeLog][Important Behavior Changes][qmake][WinRT] Cross-builds will
now ignore pre-set values of %INCLUDE% and %LIB% when building target
executables. If necessary, use configure's -I and -L switches when
building Qt, and pass QMAKE_INCDIR and QMAKE_LIBDIR on qmake's command
line when building own projects.
Reviewed-by: Thomas Miller <firstname.lastname@example.org>
Reviewed-by: Oliver Wolff <email@example.com>