Add licensing support for Gentoo Linux.

	new file:   doc/licensing/gentoo-licensing.cmake

doc/licensing/gentoo-licensing.cmake

@@ -0,0 +1,29 @@
+#
+#    Copyright 2019 Kai Pastor
+#    
+#    This file is part of OpenOrienteering.
+# 
+#    OpenOrienteering is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+# 
+#    OpenOrienteering is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+# 
+#    You should have received a copy of the GNU General Public License
+#    along with OpenOrienteering.  If not, see <http://www.gnu.org/licenses/>.
+
+
+include("linux-distribution.cmake")
+
+set(system_copyright_dir "/usr/share")
+# Pattern to find GDAL license.
+set(copyright_pattern
+  "${system_copyright_dir}/@package@/LICENSE.TXT"
+)
+
+# Location of common license text:
+set(common_license_pattern "/usr/portage/licenses/@basename@")