[tlbuild] asy2.48

[Karl Berry]

    - then adding CXXFLAGS=-std=c++11

I replaced the CXXFLAGS=-ansi suggestion for TL builds (on
tug.org/texlive/build.html).  That was for years past, when we were trying
to avoid C++11. Since now there is no avoiding it, there is no reason to
keep trying with -ansi.

So I changed build.html to suggest CXXFLAGS=-std=c++11; at least on
CentOS7, that is not the default, so it seems worth explicitly
mentioning.

    drawpath3.cc:11:23: fatal error: glm/glm.hpp: No such file or directory
     #include <glm/glm.hpp>

For TL we want to compile with opengl support if we possibly can,
since that's what John recommends.

But in principle, yes, the #include <glm/...> lines should be
conditional. John, in addition to drawpath3.cc fixed in your patch, it
looks like drawsurface.cc needs the same #ifdef.  I committed the patch
to TL, plus that change in drawsurface.cc. -k