Since all of the cryptographic primitives required by asleap (DES,
MD4, and SHA1) are obsolete, it is best to ship implementations
ourselves rather than rely on external libraries that may deprecate
them or remove them altogether.  SHA1 is already being supplied by
sha1.c, but MD4 is optionally supplied by OpenSSL (libcrypto) and for
DES we are relying on the obsolete SVID functions ‘encrypt’ and ‘setkey’.

Remove the ifdefs in common.c and genkeys.c allowing for the use of
OpenSSL’s implementation of MD4.  In the Makefile, remove both
-lxcrypt and -lcrypto from LDLIBS, and add md4.o to the link for both
asleap and genkeys.  This means the program will not compile right
now, with undefined references to ‘encrypt’ and ‘setkey’—this will be
corrected in the next commit.

While we’re messing with the Makefile, overhaul it with proper use of
object files, built-in rules and variables, and comprehensive .c:.h
dependency lists.

While we’re messing with the Makefile, eliminate the only place where
we need a -D_LINUX by instead using __GNU_LIBRARY__ to detect the
availability of <endian.h>.  This *may* mean makefile.cygwin is no
longer necessary.  Will come back to that.

This implementation was originally written by David Burren for NetBSD
and is therefore under a 3-clause BSD license.  By using it directly,
the Expand and Collapse operations in utils.c cease to be necessary.

- Crank up the warnings level (-Wextra -Wpedantic -Wstrict-prototypes)
- Explicitly request C2011 with extensions.
- Explicitly request 64-bit off_t.
- Fix all the resulting warnings.

The only code change of note is that I deleted struct dot11_mgmt from
ieee80211.h.  This was using the old GNU zero-length array extension
in a way that is not supported for C99 flexible array members, and
it’s unused in this program, so the easiest fix is to get rid of it.

The only differences are now:

 - use of .obj and .exe extensions instead of .o and null
 - -L$(WPDPACK)/lib -lwpcap instead of -lpcap
 - no ‘strip’ target