documentclass{article}
usepackage{expl3}
ExplSyntaxOn
pdftex_if_engine:TF
{
  showm
  showB
  showG
  showM
  showT
  showU
}
{
  usepackage{fontspec}
  showm
  showB
  showG
  showM
  showT
  showU
}
ExplSyntaxOff
begin{document}
% newcommand{m}{}
% newcommand{B}{}
% newcommand{G}{}
% newcommand{M}{}
% newcommand{T}{}
% newcommand{U}{}
end{document}