gl_MaxVaryingFloats was not removed from core until 4.20 and is still
available in compat shaders. Found while writing some new CTS to test
the correct declarations of this constant.
Fixes: 0ebf4257a385i ("glsl: define some GLES3 constants in GLSL 4.1")
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9514>