WebSetup.hs: use the correct --gf-lib-path when building example grammars

The directory should be the same as the one used in Setup.hs when building
the RGL, where a build directory supplied by cabal is used. This is usually
dist/build/rgl, but it could be different, e.g. when using a cabal sandbox.

WebSetup.hs

@@ -68,7 +68,7 @@ setupWeb gf args dest pkg lbi =
       where
         tmp_dir = gfo_dir</>tmp
         cmd = gf++" -make -s -optimize-pgf --gfo-dir="++tmp_dir++
-              " --gf-lib-path=dist"</>"build"</>"rgl"++
+              " --gf-lib-path="++buildDir lbi </> "rgl"++
            -- " --output-dir="++grammars_dir++  -- has no effect?!
               " "++src