Spack-installed Perl always has opcode support, but external Perl
installations might not. This commit adds a +opcode variant and
updates the external detection logic to check for opcode support.
The postgresql package is updated to require perl+opcode (in
combination with the above, this helps detect when an external
Perl instance is sufficient for a Spack build of postgreqsql, or
if Spack needs to build its own Perl).
8fdd8fcf63