Avoid substituting NAMEDATALEN, FLOAT4PASSBYVAL, and FLOAT8PASSBYVAL into

Avoid substituting NAMEDATALEN, FLOAT4PASSBYVAL, and FLOAT8PASSBYVAL into
the postgres.bki file during build, because we want that file to be entirely
platform- and configuration-independent; else it can't safely be put into
/usr/share on multiarch machines.  We can do the substitution during initdb,
instead.  FLOAT4PASSBYVAL and FLOAT8PASSBYVAL are new breakage as of 8.4,
while the NAMEDATALEN hazard has been there all along but I guess no one
tripped over it.  Noticed while trying to build "universal" OS X binaries.