#109 Use lexical binding in Emacs interfaces

[Jeronimo Pellegrini]

These files:

interfaces/emacs/emaxima/maxima-font-lock.el
interfaces/emacs/emaxima/maxima.el

do not have lexical binding enabled. I suppose dynamic binding is not necessary, and Emacs issues a warning when the files are loaded, suggesting that lexical binding be enabled for them.