I trying to build from source with the following steps (Gentoo Linux, Kernel 5.9.0):

1. git clone https://github.com/libvirt/libvirt.git
2. meson build --prefix=/home/zeno/.software/libvirt/build
3. this results in:

Program rpcgen portable-rpcgen found: NO
meson.build:927:2: ERROR: Program 'rpcgen portable-rpcgen' not found

the same happens if you run "emerge libvirt"

See also: https://github.com/libvirt/libvirt/issues/25, https://gitlab.com/libvirt/libvirt/-/issues/95