According to this post, it is stated that
To request a visual with multisampling with XLib, use these two attributes in the list to glXChooseFBConfig():
GLX_SAMPLE_BUFFERS - its value should be True. This is an on/off toggle.
GLX_SAMPLES - the number of samples.
These of which, according to the wiki on Multisampling section in opengl.org, glXChooseFBConfig() does accept these attributes. However, GLX_SAMPLE_BUFFERS/GLX_SAMPLES don't appeared to be specified in an SDK here. Do you might know why?
I suggest you download the GLX-1.4 specification PDF, which is the much better reference manual: http://www.opengl.org/registry/doc/glx1.4.pdf