glx: Build GLX normally.

glx: Build GLX normally.

GLX was built specially and before Mesa core because libglapi.a could
not be built with IN_DRI_DRIVER defined.  This is no longer the case
since 6e99e6ddbf488f6955e34ef0bc438fdcb4d90f74.

It works fine in my (limited) testing with both direct and indirect
rendering.  I also compare the outputs after preprocessing (gcc -E) with
or without this commit, and they are identical.