Replace -W -Wall hardcoded into a bunch of Makefiles with $(GCCWARN),
a centralized variable defined in the root MCONFIG.  Add
-Wstrict-prototypes to the list of global warnings: we should never
have non-prototyped declarations.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

Make the dependency generation more common; have a general pattern in
MCONFIG, and use it in rules (not in CFLAGS).

For NASM source, in order to stay compatible with old versions of
NASM, run NASM twice; newer versions of NASM is capable of generating
dependencies simultaneously like gcc can, but that would break
compatibility with older distros.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

Cleanup and centralize the Makefile system even more.
Fix a gcc 4.3 incompatibility in memdisk (definition of strlen).

Clean up the BSUBDIR/ISUBDIR separation further.  Build _bin.c files,
which are an intermediate stage toward building the installers, in the
libinstaller directory, since that directory is used by all the
installers anyway.  That also lets us get bin2c.pl out of the root.

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.

Assemble all files to ELF instead of using the NASM built-in linker.
This can enable us to do actual linking in the future.

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

--once now supported for extlinux; some infrastructure for syslinux
there as well, need implementation in all the various installers,
however.

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.