x11/libXxf86dga: Update to 1.1.5

Update x11/libXxf86dga to 1.1.5
This release raises the minimun libX11 version 1o 1.6.0
it also contains various build system fixes

add LICENSE_FILE
pet portlint by moving USES= earlier
add INSTALL_TARGET=install-strip to strip installed binaries and libraries

Changelog:
https://lists.x.org/archives/xorg-announce/2019-March/002982.html

Sponsored by: B3 Init