refactor CMake

- allow compiling without microhttpd
- allow compiling without OpenSSL
- install `config.txt` with command `make install`

httpd.cpp

@@ -21,6 +21,8 @@
   *
   */
 
+#ifndef CONF_NO_HTTPD
+
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -142,3 +144,5 @@ bool httpd::start_daemon()
 	return true;
 }
 
+#endif
+