Most installations of OpenSSL come with a system wide directory of default certificate authorities and most applications using OpenSSL make use of this certificate store during server certificate verification, which is handy because a system administrator only has to manage one central store of acceptable certificate authorities.

Fossil presently doesn't use the system wide CA certificate store, but I think it should :-) The necessary change is only one line of code:
<verbatim>
Index: src/http_ssl.c
===================================================================
--- src/http_ssl.c
+++ src/http_ssl.c
@@ -90,10 +90,11 @@
     SSL_library_init();
     SSL_load_error_strings();
     ERR_load_BIO_strings();
     OpenSSL_add_all_algorithms();
     sslCtx = SSL_CTX_new(SSLv23_client_method());
+    X509_STORE_set_default_paths(SSL_CTX_get_cert_store(sslCtx));
     sslIsInit = 1;
   }
 }

 /*
</verbatim>