Make function mbedtls_ssl_set_hostname(...) as optional

Now function mbedtls_ssl_set_hostname is compile-time configurable
in config.h with define MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION.
This affects to many x509 API's. See config.h for details.

doxygen/mbedtls.doxyfile

@@ -1624,7 +1624,8 @@ PREDEFINED             = WIN32 \
                          ENABLE_PLUGIN \
                          ENABLE_MANAGEMENT \
                          ENABLE_OCC \
-                         HAVE_GETTIMEOFDAY
+                         HAVE_GETTIMEOFDAY \
+                         DOXYGEN_ONLY
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then
 # this tag can be used to specify a list of macro names that should be expanded.