dblink and postgres_fdw use SHLIB_PREREQS = submake-libpq to build libpq
first. This doesn't work in a PGXS build, because there is no libpq to
build. So just omit setting SHLIB_PREREQS in this case.
Note that PGXS users can still use SHLIB_PREREQS (although it is not
documented). The problem here is only that contrib modules can be built
in-tree or using PGXS, and the prerequisite is only applicable in the
former case.
Commit 6697aa2bc25c83b88d6165340348a31328c35de6 previously attempted to
address this by creating a somewhat fake submake-libpq target in
Makefile.global. That was not the right fix, and it was also done in a
nonportable way, so revert that.
1e95bbc870