BuildSystem: Detect ca_list_path capabilities in external PJProject.

Since Asterisk 13.8, pj_ssl_cert_load_from_files2 got detected only in the
bundled PJProject but not in an external PJProject. Therefore, ca_list_path
could not be used in pjsip.conf. With this change, pj_ssl_cert_load_from_files2
is detected again to enable ca_list_path again.

ASTERISK-26303 #close

Change-Id: I4a4a0cdc5cdff33730911fb4cfc0498c069043d0