"system" refers to the system's native shell, which is what qmake's
system() invokes, and whose convention by far most commands invoked from
a makefile will need.
"shell" refers to the shell invoked by make, which diverges from the
system shell only when qmake/mingw32-make is called from an msys shell.
its conventions need to be used for anything the shell itself does
(e.g., assembling env variables, but also command line argument
unquoting) and the commands the mkspec sets according to the shell
(e.g., QMAKE_MOVE).

Change-Id: I0000aa9417c199cf8a810619d31ded24bb0675f9
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>