ppl011: disable demo programs
ppl011: disable demo programs

This fixes the build in the presence of glpk from homebrew-science

Fixes #326.

UniqMartin/homebrew-versions,DomT4/homebrew-versions,heuermh/homebrew-versions,jimhe/homebrew-versions,coeligena/homebrew-verscustomized,Ashton-W/homebrew-versions