lib/egif_lib.c : fix cross compilation to windows : '\' is not usable on linux and '/' is usable on Windows. the other headers exist on Windows so no hurt if we include them
for Windows compilation (cross compile or not), AC_LIBTOOL_WIN32_DLL is needed (see autoconf manual)
use silent rule for better make output
build doc only if xmlto is found (otherwise, make fails)
-no-undefined can be (and should be) used also on unix (simpler code)
* -lm directly added. If a system don't have libm, it will probably not be able to compile giflib.
other files reflects the changes in configure.ac