lyx: updated to version 2.1.2

Modified some dependencies libmagic wasn't needed file-devel
was needed at build time and GraphicsMagick at run time as
it is used by the program to convert between file formats.

Modified also the INSTALL.msg to make it easy copy and paste
the needed steps to expand the TeXLive default installation
to suit the LyX needs.

Regards

carlo D.
Merge pull request #636 from onekk/master

lyx: updated to version 2.1.2