This week Marek Olšák introduced a GALLIUM_MSAA environment variable for setting the multi-sample anti-aliasing level. It works as one would suspect by setting the desired sample count as the value in the GALLIUM_MSAA environment variable or zero to have the support disabled.
Aside from the GALLIUM_MSAA variable, the __GL_FSAA_MODE environment variable serves the same purpose and is supported by this week's Git activity. The __GL_FSAA_MODE option is what NVIDIA's driver utilizes and thus there is Gallium3D support for it to try to provide some level of standardization.
Marek tested this support against Enemy Territory: Quake Wars for Linux where there is no native MSAA anti-aliasing controls. The commit to mainline Mesa can be found here.