Partly fixes <http://bugs.gnu.org/18002>.
Reported by David Thompson <dthompson2@worcester.edu>.
* guix/gexp.scm (gexp->derivation): Change #:system to default #f.
Use (%current-system) from within the 'mlet*'.
* tests/gexp.scm ("gexp->derivation, default system"): New test.
* tests/gexp.scm (shebang): New variable.
Skip "gexp->script" when SHEBANG is longer than 127 chars.
* guix/gexp.scm (gexp->script): Add comment on the issue.
* guix/gexp.scm: New file.
* tests/gexp.scm: New file.
* Makefile.am (MODULES): Add guix/gexp.scm.
(SCM_TESTS): Add tests/gexp.scm.
* doc/guix.texi (Derivations): Add #:inputs in 'derivation' example.
Mark 'build-expression->derivation' as deprecated, refer to
"G-Expressions". Remove paragraph about code strata.
(G-Expressions): New node.