Disable SYSTEM_COMPILER by default.

This is a direct commit to releng/11.0.

Having it enabled can lead to a situation where building
on one system and installing on another will fail due
to not finding cc in the OBJDIR.

An actual fix will be made on head separately.

PR: 212877
Relnotes: yes
Sponsored by: Dell EMC Isilon
Approved by: re (gjb)