x11-toolkits/libXmu: Update to 1.1.3

Update x11-toolkits/libXmu to 1.1.3
This is a minor maintenance release with small bug fixes, build
configuration improvments and janitorial cleanups.

add INSTALL_TARGET=install-strip to strip binaries and libraries

Changelog:
https://lists.x.org/archives/xorg-announce/2019-March/002978.html

Sponsored by: B3 Init