built-in-constants: don't try to create GS with version < 150.

Previously, built-in-constants looked at required_glsl_version to
determine what to include in the "#version" directive, but it looked
at glsl_version (the maximum version supported by the implementation)
to decide whether to test geometry shaders.  As a result, it when
testing a built-in constant whose required_glsl_version was less than
150, it would try to create a geometry shader using a "#version"
directive less than 150, leading to GL errors.

This patch prevents the problem by only testing geometry shaders when
required_glsl_version >= 150.