glsl: fix declarations of gl_MaxVaryingFloats

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>

src/compiler/glsl/builtin_variables.cpp

@@ -772,10 +772,12 @@ builtin_variable_generator::generate_constants()
          add_const("gl_MaxDualSourceDrawBuffersEXT",
                    state->Const.MaxDualSourceDrawBuffers);
       }
-   } else {
-      /* Note: gl_MaxVaryingFloats was deprecated in GLSL 1.30+, but not
-       * removed
-       */
+   }
+
+   /* gl_MaxVaryingFloats was deprecated in GLSL 1.30+, and was moved to
+    * compat profile in GLSL 4.20. GLSL ES never supported this constant.
+    */
+   if (compatibility || !state->is_version(420, 100))  {
       add_const("gl_MaxVaryingFloats", state->ctx->Const.MaxVarying * 4);
    }