If ${DESTDIR} was non-empty and the source file was an absolute
path, the postinstall check target defined in mk/pkgtasks/files.mk
was improperly failing. Fix the bug by only prepending ${PREFIX}
if the source path is relative, and always prepending ${DESTDIR}.
Add definitions for the gtk-update-icon-cache(1) command to the
script header template, and a makefile fragment that creates the
data file for the "icon_themes" package task if ${ICON_THEMES} is
"yes" in the package Makefile.
Also bump pkgtasks dependency to version 1.12 for the "icon_themes"
package task and compatibility with pkgsrc/mk/pkginstall.
Add missing feature from "pkginstall" framework where the config
directory ${PKG_SYSCONFDIR} was automatically created if one of
the following conditions were true:
* PKG_SYSCONFSUBDIR was defined and non-empty.
* Any configuration files were copied into ${PKG_SYSCONFDIR}.
A full dependency on the "pkgtasks" package will be added if a
non-empty data file is generated by the framework.
This framework is not yet hooked into bsd.pkg.mk, pending some
other changes.
