[CMake] Suggesting to FindQT4.cmake where to find QT4

[Jed Brown]

David Thompson wrote:
>>> Have you ever tried setting the PATH?  It is actually quite easy and does
>>> not involve "great contortions".
>>>
>>> All I do is execute
>>>
>>> PATH='/home/software/qtsdk-2009.02/qt/bin:'$PATH
>>>
>>> and from then on, cmake uses the version of Qt4 that I downloaded and
>>> installed in the specified location.  If I want the system version of Qt4
>>> again (which I don't for my needs), I remove the above from my PATH.
>> The issue is obviously what happens as soon as there are two independent
>> libraries that are both so selfish.  Reylying on PATH being set up
>> specially for you is terrible design unless you can guarantee that there
>> will never be another.
> 
> Perhaps there is a miscommunication here; I think Alan is assuming that
> you install at least one (if not all) versions of Qt into their own
> directories (with no other software
> in /home/software/qtsdk-2009.02/qt/bin except Qt) while you are assuming
> that multiple packages are being installed into the same directory tree.
> Doing things the former way, you can have as many of these entries in
> your path as you like and there will never be a conflict among packages
> because each PATH entry only points to a single package.

That's fine as long as all your users, with their wide variety of
(dis-)organization schemes, agree that this is how software is
installed.  The moment that is not true, this scheme breaks down.  With
autoconf, for example, you would normally be able to specify
--with-foo-dir=/path/a --with-bar-dir=/path/b.  I don't see why there is
so much resistance to making this sort of thing work in a reliable and
consistent manner.

Jed

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