dri: make unresolved symbol test link work even without a libGL.so

dri: make unresolved symbol test link work even without a libGL.so

Currently the test link uses -lGL to define the glapi symbols.

This makes it impossible to build DRI drivers on systems without
Mesa installed and without building the libGL from the Mesa tree
first.

Some automated build systems trigger this problem.

This commit removes -lGL and instead adds a dummy implementation of
glapi to dri_test.c

This, along with Kristian's commit, should fix all known regressions
due to the addition of unresolved symbol checking.