libxmu-dev binary package in Ubuntu Focal riscv64

 libXmu provides a set of miscellaneous utility convenience functions for X
 libraries to use. libXmuu is a lighter-weight version that does not depend
 on libXt or libXext; for more information, see libxmuu1.
 .
 This package contains the development headers for the library found in
 libxmu6. Non-developers likely have little use for this package.
 .
 More information about X.Org can be found at:
 <URL:http://www.X.org>
 .
 This module can be found at
 git://anongit.freedesktop.org/git/xorg/lib/libXmu

Source package