rewrite qtAddToolEnv()

the primary purpose is making env var prepend mode work for unset
variables on windows. this is achieved by using a conditional and delayed
variable expansion. however, the latter is disabled by default and can
be locally enabled only in batch files. therefore, write wrapper scripts
and substitute them for the actual commands. we do this also on unix,
both for consistency and simply because the commands look much less
confusing.

this change is slightly backwards-incompatible, as invoking
qtAddToolEnv() multiple times on the same command will now make a total
mess. also, invoking it on a command that contains 'make' macro
expansions isn't a good idea, so testcase.prf needed an adjustment. the
function is an undocumented internal, so Nobody Should Care (TM).

this also reverts 80ebedec

, as it's obsolete now.

Change-Id: I8394b77868b495abcf27b688996ca74c40b80994
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>