packages: Add `native-search-paths' field and honor it.

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.