main: Warn that geometry shader support is experimental.

Geometry shader support in the Mesa front end is still fairly
preliminary.  Many features are untested, and the following things are
known not to work:

- Redeclaring built-in inputs using the gl_in interface block
- The gl_ClipDistance input
- Transform feedback of geometry shader outputs
- Constants that are new in GLSL 1.50 (e.g. gl_MaxGeometryInputComponents)

This isn't a problem, since no back-end drivers currently enable
geometry shaders.  However, to make sure no one gets the wrong
impression, emit a nasty warning to let the user know that geometry
shader support isn't complete.