settings: don't hard-code HTTPS capability

This partially reverts bdec62dc which activates
the transport code-paths which allow you to use a custom TLS implementation
without having to have one at build-time.

However the capabilities describe how libgit2 was built, not what it could
potentially support, bring back the ifdefs so we only say we support HTTPS if
libgit2 was itself built with a TLS implementation.

src/settings.c

@@ -29,7 +29,9 @@ int git_libgit2_features(void)
 #ifdef GIT_THREADS
 		| GIT_FEATURE_THREADS
 #endif
+#if defined(GIT_OPENSSL) || defined(GIT_WINHTTP) || defined(GIT_SECURE_TRANSPORT)
 		| GIT_FEATURE_HTTPS
+#endif
 #if defined(GIT_SSH)
 		| GIT_FEATURE_SSH
 #endif