packages: Add `native-search-paths' field and honor it.
* guix/packages.scm (<search-path-specification>): New record type.
(search-path-specification->sexp): New procedure.
(<package>)[native-search-paths]: New field.
(package-derivation): Accumulate the search paths, and pass them
as #:search-paths toe BUILDER.
* guix/build-system/gnu.scm (gnu-build): Add #:search-paths. Compute
`implicit-search-paths'. Pass #:search-paths in BUILDER.
* guix/build-system/perl.scm (perl-build): Add #:search-paths, pass it
to BUILDER with the search paths of PERL.
* guix/build-system/cmake.scm (cmake-build): Add #:search-paths, pass it
to BUILDER.
* guix/build-system/trivial.scm (trivial-build): Add #:search-paths,
ignore it.
* guix/build/gnu-build-system.scm (set-paths): Add #:search-paths.
Remove explicit settings of CPATH, LIBRARY_PATH, and PKG_CONFIG_PATH.
Instead, walk SEARCH-PATHS and call `set-path-environment-variable'
for them.
* guix/build/perl-build-system.scm (perl-build): Remove PERL5LIB setting.
* tests/packages.scm ("search paths"): New test.
* gnu/packages/bootstrap.scm (%bootstrap-guile)[raw]: Add
#:search-paths.
(%bootstrap-gcc): Add `native-search-paths' field.
* gnu/packages/perl.scm (perl): Likewise.
* gnu/packages/pkg-config.scm (pkg-config): Likewise.
* gnu/packages/glib.scm (intltool): Remove `arguments'.
* gnu/packages/avahi.scm (avahi): Remove #:phases.
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