gnu: Avoid circular dependencies by Perl license.

gnu: Avoid circular dependencies by Perl license.

* guix/licenses.scm (perl-license): New variable.
* gnu/packages/bioinformatics.scm: Change (package-license perl) to perl-license.
* gnu/packages/compression.scm: Same.
* gnu/packages/databases.scm: Same.
* gnu/packages/gd.scm: Same.
* gnu/packages/language.scm: Same.
* gnu/packages/libevent.scm: Same.
* gnu/packages/mail.scm: Same.
* gnu/packages/messaging.scm: Same.
* gnu/packages/ncurses.scm: Same.
* gnu/packages/networking.scm: Same.
* gnu/packages/photo.scm: Same.
* gnu/packages/tcl.scm: Same.
* gnu/packages/tex.scm: Same.
* gnu/packages/tls.scm: Same.
* gnu/packages/web.scm: Same.
* gnu/packages/wm.scm: Same.
* gnu/packages/xml.scm: Same.
* gnu/packages/xorg.scm: Same.
* gnu/packages/zip.scm: Same.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>