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)

1 files changed, 1 insertions, 2 deletions

diff --git a/demos/smime/smver.c b/demos/smime/smver.c
index 2e55c72584..caa9ed304f 100644
--- a/demos/smime/smver.c
+++ b/demos/smime/smver.c
@@ -66,10 +66,9 @@ int main(int argc, char **argv)
         goto err;
     }
 
-    fprintf(stderr, "Verification Successful\n");
+    printf("Verification Successful\n");
 
     ret = EXIT_SUCCESS;
-
  err:
     if (ret != EXIT_SUCCESS) {
         fprintf(stderr, "Error Verifying Data\n");