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.
Change-Id: I36f53e8880d6523f3f6f7a44d40d87d04bd06854
Reviewed-by: Thomas Miller <thomaslmiller91@gmail.com>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
