demos: tidy up makefiles, fix warnings

demos: tidy up makefiles, fix warnings

Update makefiles so that consistent patterns are used.  Object files
are compiled from source using an implicit rule (but using our
CFLAGS); for linking, we give an explicit rule.  Ensure that "make
test" works in each subdirectory (even if it does not actually run any
applications).  The top-level demo makefile now works.

The makefiles are not make-agnostic.  e.g. they use the variable $(RM)
in "clean" recipes, which is defined in gnu-make but may not be
defined in others.

Part of #17806

Testing:

  $ cd demo
  $ make test

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22698)