Now we have just one library of "all of Mesa core" instead of both
libdricore and libglsl that drivers link against.
I did this change in a sort of nonrecursive make fashion: the
generated files are still produced in the non-automake build, like the
rest of dricore, but the GLSL files are stuffed into libdricore
without building a convenience library in src/glsl (even though we
could now). This would make a bit more sense if glsl was just another
dir under src/mesa, because right now I had to contort the prefix
variable name to look another ../ level up.