Search paths

[Ludovic Courtès]

Hey, hey!

Commit a18eda2 in ‘core-updates’ adds the ‘native-search-paths’ field
for packages, and honor it.  The next commit gives an example of how to
use it.

As discussed before, this mechanism allows the declaration of a
package’s search path environment variables.  Examples follow.

GCC has this:

     (native-search-paths
      (list (search-path-specification
             (variable "CPATH")
             (directories '("include")))
            (search-path-specification
             (variable "LIBRARY_PATH")
             (directories '("lib" "lib64")))))

Guile 2.0 has this:

   (native-search-paths
    (list (search-path-specification
           (variable "GUILE_LOAD_PATH")
           (directories '("share/guile/site/2.0")))
          (search-path-specification
           (variable "GUILE_LOAD_COMPILED_PATH")
           (directories '("share/guile/site/2.0")))))

Perl has this:

    (native-search-paths (list (search-path-specification
                                (variable "PERL5LIB")
                                (directories '("lib/perl5/site_perl")))))

With those specifications, any package that has (say) Perl as an input
along with several Perl modules has PERL5LIB correctly set in its build
environment.

It’s extensible, so it’s much better than what we had before.

Please report any problems or kudos!  :-)

Thanks,
Ludo’.