Log Message:
Update TeXmacs to 1.99.5.

Explicitly does not support guile20, so let it use guile18 for now.

Changes from version 1.99.3 to 1.99.5
    Support for retina screens (1.99.5).
    Implementation of a better, still experimental, page breaking algorithm (1.99.5).
    Various improvements for the laptop presentation mode (1.99.5).
    Simplified editor for graphical slides in laptop presentations (1.99.5).
    Highly improved animations (1.99.5).
    An editing mode for simple animations based on morphing (1.99.5).
    Various graphical effects and font effects (1.99.5).
    Reorganizations in the configuration of TeXmacs (1.99.5).
    Improved rendering of pages, including a mode for double sided rendering, \ 
and a panorama mode (1.99.5).
    High quality mathematical typesetting for many standard fonts (1.99.5); see \ 
also the paper “Mathematical Font Art” presented at ICMS 2016.
    Rudimentary support for inking (1.99.5).
    Customizable snapping in graphics mode (1.99.5).
    Various enhancements and fixes for graphics mode (1.99.5).
    Various bug fixes (1.99.4).
    Various improvements for the table editor (1.99.4).
    Greek language support, with the help of Alkis Akritas (1.99.4).
    Experimental math editing mode that enforces syntactic correctness (1.99.3).
    A series of improvements in the converter to LaTeX (1.99.3).
    Tool for examining errors in the LaTeX export (1.99.3).
    High quality support of STIX fonts (1.99.3).
    Various improvements concerning mathematical typesetting (1.99.3).
    Improved punctuation, including support for French punctuation rules (1.99.3).
    Improved experimental client-server support (1.99.3).
    Encrypting/decrypting pieces of documents (1.99.3).
    New experimental bibliographic database management (1.99.3).
    New experimental database facilities (1.99.3).
    Added style package for literate programming (1.99.3).
    Consistent support for multiple bibliographies, indexes, glossaries, etc. \ 
(1.99.3).