Rewrite link time optimization in Makefile

Rewrite link time optimization in Makefile

Instead of binding link time optimization to the choice of
popcount support, do the right thing and add -flto option
when gcc 4.5 or later is detected.

Although it should be supported also under mingw, it happens
that it doesn't, at least on my 4.6.1 due to some known bugs.

Thanks to Mike for helping me with this patch.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>