This build option was only useful to maintain compatibility for
version-0 functions, but those are no longer supported, so this option
can be removed.

float4 is now always pass-by-value; the pass-by-reference code path is
completely removed.

Discussion: https://www.postgresql.org/message-id/flat/f3e1e576-2749-bbd7-2d57-3f9dcf75255a@2ndquadrant.com