* Makefile.in: Add `target_os' and `enable_grub_pe2elf'.
* conf/common.rmk: Install `grub-pe2elf' only if requested.
Install `grub.d/10_windows' only on Cygwin.
* configure.ac: Add subst of `target_os'.
Check `target_os' also before setting TARGET_OBJ2ELF.
Add `--enable-grub-pe2elf'.