error in MSVC incremental link command

On some Windows installations Qt's configure fails with the error message

NMAKE : fatal error U1077: 'if' : return code '0x2'

after building qmake.

The culprit is in the generated Makefile of config.tests/arch

if not exist $(DESTDIR_TARGET) del arch.exe.embed.manifest>NUL 2>&1

On some machines del exits with an error code.
In those cases the build will fail.

We must not rely on del never setting ERRORLEVEL.