mingw: use ASLR, no-SEH and DEP if available

If the linker supports the flags --dynamicbase, --no-seh,
or --nxcompat, use them.

Tested on Windows Vista: Process Explorer reports that ASLR and DEP
are in use. No effect seen on Wine or Windows XP.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>