glsl: Add parser support for GL_ARB_explicit_attrib_location layouts

Only layout(location=#) is supported.  Setting the index requires GLSL
1.30 and GL_ARB_blend_func_extended.
diff --git a/src/glsl/glsl_parser_extras.cpp b/src/glsl/glsl_parser_extras.cpp
index 26e0721..844a746 100644
--- a/src/glsl/glsl_parser_extras.cpp
+++ b/src/glsl/glsl_parser_extras.cpp
@@ -181,6 +181,13 @@
 	 state->ARB_draw_buffers_enable = (ext_mode != extension_disable);
 	 state->ARB_draw_buffers_warn = (ext_mode == extension_warn);
       }
+   } else if (strcmp(name, "GL_ARB_explicit_attrib_location") == 0) {
+      state->ARB_explicit_attrib_location_enable =
+	 (ext_mode != extension_disable);
+      state->ARB_explicit_attrib_location_warn =
+	 (ext_mode == extension_warn);
+
+      unsupported = !state->extensions->ARB_explicit_attrib_location;
    } else if (strcmp(name, "GL_ARB_fragment_coord_conventions") == 0) {
       state->ARB_fragment_coord_conventions_enable =
 	 (ext_mode != extension_disable);