multimedia/mxflib: Limit portscout to numeric versions

SourceForge still carries mxflib-1.0.1-gcc4.3.tar.gz, a gcc 4.3 build fix
respin of the same 1.0.1 release. Portscout reads the -gcc4.3 suffix as a
version component and sorts it above 1.0.1, reporting a permanent false
positive.