Avoid crashing in the presence of unnamed submenus.

Move source files out of the root directory; the root is a mess and
has become virtually unmaintainable.  The Syslinux core now lives in
core/; the Linux and generic utilities has moved into utils/, and
copybs.com has moved into dos/; it had to go somewhere, and it seemed
as good a place as any.

Use $(CC) in gcc_ok macro, not plain gcc.  This seems to work, iff the
gcc_ok macro is declared with =, not :=

Create NASMOPT variable, defaulting to -O9999.  Mostly there to test
beta versions of NASM.

Compiling as root is highly discouraged, but some people do it anyway.
gcc_ok, however, can clobber /dev/null due to "-o /dev/null"; this is
bad.  Instead, write a temporary file and delete it.