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.

If the variable GZIP exists in the environment, overriding it in the
Makefile exports it into the environment, causing failures:

gzip -cd syslogo.ppm.gz | \
		../ppmtolss16 \#000000=0 \#d0d0d0=7 \#f6f6f6=15	\
		> syslogo.lss
gzip.gz: No such file or directory
204480 pixels, 25715 bytes, (74.85% compression)

Use $GZIPPROG instead.

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

Ship syslogo.ppm.gz instead of syslogo.png, thereby eliminating a
dependency on pngtopnm which apparently isn't installed by default on
some distros.

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

Small Perl script to postprocess the listfile and show absolute addresses
instead of section-relative.  This only applies to the main address, not
to addresses in bracketed fields.

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.