CMake: allow compile-time C++ flags to be set

For developper only, therefore the variable is set to advanced.

Ideas from: Scott Kostyshak + Vincent van Ravensteijn
Fix opening files from command line

Fixes: 9c4461de
ar.po: update from Hatim
Add some missing unicode math symbols

and fix wrong ones. This fixes the safe part of bug #8888. The symbols
provided by mdsymbol.sty have to wait, since mdsymbol.sty provides a huge
number of symbols, I don't have the time right now to process them all, and
a partial file format update does not make sense.
Make sty file parser fit for mdsymbol.sty

Now it is able to generate a lib/symbols section from mdsymbol.sty.
Fix writer2latex quote handling (bug #8903)

writer2latex surrounds quotes by braces which we skip to avoid useless ERT.
I broke this in 25fe87e5 which made Parser::next_next_token() not recognize
that it needed to parse one more token. If we had a unit test for the Parser
class it would probably have detected that. Now we have at least a test for
the special quote.
Add appdata.xml, provided by Matej Cepl.

AppData is an emerging standard (hopefully not distro-specific) for the
description of applications in various app stores.

http://people.freedesktop.org/~hughsient/appdata/ is the home page of
the standard.

http://blogs.gnome.org/hughsie/2013/09/27/frikin-awesome-apps-without-appdata/
is the call for providing new appdata.xml files.
ctests: non-TeX Ukrainian+LuaTeX is not supported

https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg181690.html

Thanks to Günter Milde for the advice.
ctests: use Chinese roman font for zh docs
ar.po: update from Hatim
Sharpen the lyX icon a bit
Improve beamer conversion: Support default list overlay specification without ERT
Do not load the tipa package when fontspec is used

This was until now only cared of by the IPA inset, not by unicodesymbols.
beamer.lyx: work arounf language nesting problem when compiling with polyglossia
Recognize changed babel error message (rewording)
Fix typo
Adapt cmake build to use c++11 features also for clang compiler
Fix typo
ar.po: Update from Hatim
Cmake tests: Expand export tests for luatex and xetex

Citing Scott:
In our current set up, we are currently testing XeTeX and LuaTeX
either with system fonts or with TeX fonts but never both. We should
test with both in my opinion. We will have to ignore/invert many tests
but it still seems useful. For example Günter fixed babel-greek so
that it works now with TeX fonts; and Jürgen found some errors in LyX
that were causing some of the English docs to fail with system fonts.
Currently we only test greek documents with system fonts and we only
test English documents with TeX fonts.

This change adds the missing test-cases.