Fix bug #6469: Times Roman and \textgreek produce error with pdflatex on a PC

Fix bug #6469: Times Roman and \textgreek produce error with pdflatex on a PC
The problem here is that the metafont sources and tfm files are missing
in normal installations, because the Times Roman Greek fonts are not free
and should be purchased. When the grtimes package is not installed, the
fonts are substitued with cmr glyphs. However, if grtimes is installed
and and the actual fonts are missing, an error is generated. So, we simply
check whether the greek fonts are installed and, if they are not, we
perform the font substitution by ourselves. In this way the error is
avoided whether grtimes is installed or not.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@33169 a592a061-630c-0410-9148-cb99ea01b6c8