[bug-sourceinstall] --prefix option not retained

[Nicola Fontana]

Hi,

while upgrading packages I found another usability issue.
Following the golden rule of the less-surprise, I expect to do
the following:

1. Choose the package
2. Click upgrade
3. Select the tarball or source directory
4. Confirm the operation
5. Leave the old configuration options and start the build

The problem is in 5.: all the configuration options are properly
retained, all but the --prefix one, overwrote by its global
value.

Having global option values is a great feature but while
upgrading I think the old options must have a higher precedence,
so once every package is properly installed, upgrading the whole
system will be a piece of cake.

I attached a patch (not deeply tested) that does the job.

While writing it I noticed something weird on the parser that
gets the old values. There is a single strstr() that searches
for the option name: what about if I have two arguments such as
--with-x-include and --with-x? Will be the --with-x old value
ignored in this case?

Ciao
-- 
Nicola

pre-fix.patch
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