pkg-config Requires.private entries should be comma-separated

pkg-config Requires.private entries should be comma-separated

In the .pc (pkg-config) files generated by the make and meson builds,
the Requires.private entries use different delimiters.  The make build
uses spaces, the meson build uses commas. The pkg-config documentation
says that it should be comma-separated, but apparently about half the
.pc in the wild use just spaces.  The pkg-config source code
acknowledges that both commas and spaces work.

This changes the make build to use commas, for consistency.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/1fb52d61-0964-2d8e-87d9-e8be830e2b24%40enterprisedb.com