Disabling link with already installed libraries

[vincent habchi]

 From #22200 :

> thought it was worth noting that there is an additional problem.  
> no_atlas
> variant has no effect if the atlas package is installed - numpy is  
> linked
> against atlas regardless. This seemed to me a different issue than the
> above, hence the two different tickets.

That's something I experienced also with other ports, notably when  
they are using Cmake. They find libraries in ${prefix}, wherever the  
user has enabled or disabled the corresponding option.

Being familiar with another build-from-source-framework, namely pkgsrc  
(wherefore I install my packages in /usr/pkg and not /opt) on NetBSD,  
it uses a more subtile process: all the dependencies are linked in a  
temporary repertory, and the traditional tools like gcc, g++, etc. are  
replaced by shell scripts that provide on the fly substitutions of  
library and include paths from standard directories to that temporary  
one. Therefore:

1. When a dependency is not present, the package does not compile;
2. If an option flag that pulls a library in is not set, the library  
is not symlinked and the configure script does not find it, thereby  
enforcing the lack of dependency.

Is there any hope to get such a behavior in Macports?

Vincent