This directive is available from 2.5 on. For 2.4 
it syntax check fails with following way:

SKIPPED
AH00526: Syntax error on line 19 of /tmp/apache-rex/core-Error-basic/httpd.conf:
SSL flag defined, but ssl module not loaded

For 2.5 syntax check fails with:

SKIPPED
[Fri May 13 08:57:27.998872 2016] [core:error] [pid 17758] SSL flag defined, but ssl module not loaded on line 19 of /tmp/apache-rex/core-Error-basic/httpd.conf
AH00526: Syntax error on line 19 of /tmp/apache-rex/core-Error-basic/httpd.conf:
Configuration processing stopped by Error directive

