remove automatic splitting from $$() expansions

$$(FOO) would automatically split the contents of the environment
variable at whitespace (and interpret quoting inside it). the way to
prevent the splitting (but not the quote interpretation) would be using
"$$(FOO)".

this behavior is entirely unexpected and thus an incredibly effective
source of quoting problems - according to a grep over the whole qt
sources, there isn't a single case where things were done right. in qt
creator, well over half the cases are wrong.

also, the "feature" seems entirely pointless: nobody uses spaces as
separators in environment variables.

consequently, simply remove it, even in a patch release. i'm postulating
that nobody will complain.

Change-Id: I9ed3df1b0d1ef602acd78ceb118611d294561da6
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from qtbase/566d28316dd73bfe4a8ea34839989ee9df7e3895)