lib: skip compiling code with features disabled

In non-SSL, non-SSH, non-H3, non-AppleSecTrust builds.

Also:
- drop unused internal macro `SSL_SYSTEM_VERIFIER`.

Closes #20587

lib/vssh/ssh.h

@@ -24,6 +24,7 @@
  *
  ***************************************************************************/
 #include "../curl_setup.h"
+#include "../urldata.h"
 
 extern const struct Curl_protocol Curl_protocol_sftp;
 extern const struct Curl_protocol Curl_protocol_scp;
@@ -31,6 +32,8 @@ extern const struct Curl_protocol Curl_protocol_scp;
 extern const struct Curl_scheme Curl_scheme_sftp;
 extern const struct Curl_scheme Curl_scheme_scp;
 
+#ifdef USE_SSH
+
 #ifdef USE_LIBSSH2
 #include <libssh2.h>
 #include <libssh2_sftp.h>
@@ -236,8 +239,6 @@ struct ssh_conn {
 
 #endif /* USE_LIBSSH2 */
 
-#ifdef USE_SSH
-
 #ifdef CURLVERBOSE
 const char *Curl_ssh_statename(sshstate state);
 #else
@@ -255,11 +256,10 @@ void Curl_ssh_cleanup(void);
 void Curl_ssh_version(char *buffer, size_t buflen);
 void Curl_ssh_attach(struct Curl_easy *data,
                      struct connectdata *conn);
-#else
-/* for non-SSH builds */
+#else /* !USE_SSH */
 #define Curl_ssh_cleanup()
 #define Curl_ssh_attach(x, y)
 #define Curl_ssh_init() 0
-#endif
+#endif /* USE_SSH */
 
 #endif /* HEADER_CURL_SSH_H */

lib/vssh/vssh.c

@@ -22,10 +22,11 @@
  *
  ***************************************************************************/
 #include "../curl_setup.h"
+#include "ssh.h"
+
+#ifdef USE_SSH
 
 #include "vssh.h"
-#include "ssh.h"
-#ifdef USE_SSH
 #include "../curlx/strparse.h"
 #include "../curl_trc.h"
 #include "../escape.h"

lib/vssh/vssh.h

@@ -25,6 +25,8 @@
  ***************************************************************************/
 #include "../curl_setup.h"
 
+#ifdef USE_SSH
+
 #include "../urldata.h"
 
 CURLcode Curl_getworkingpath(struct Curl_easy *data,
@@ -36,5 +38,5 @@ CURLcode Curl_get_pathname(const char **cpp, char **path, const char *homedir);
 CURLcode Curl_ssh_range(struct Curl_easy *data,
                         const char *range, curl_off_t filesize,
                         curl_off_t *startp, curl_off_t *sizep);
-
+#endif /* USE_SSH */
 #endif /* HEADER_CURL_VSSH_VSSH_H */