oauth: Correct missing comma in Requires.private

oauth: Correct missing comma in Requires.private

I added libcurl to the Requires.private section of libpq.pc in commit
b0635bfda, but I missed that the Autoconf side needs commas added
explicitly. Configurations which used both --with-libcurl and
--with-openssl ended up with the following entry:

    Requires.private: libssl, libcrypto libcurl

The pkg-config parser appears to be fairly lenient in this case, and
accepts the whitespace as an equivalent separator, but let's not rely on
that. Add an add_to_list macro (inspired by Makefile.global's
add_to_path) to build up the PKG_CONFIG_REQUIRES_PRIVATE list correctly.

Reported-by: Wolfgang Walther <walther@technowledgy.de>
Reviewed-by: Fabrízio de Royes Mello <fabriziomello@gmail.com>
Discussion: https://postgr.es/m/CAOYmi+k2z7Rqj5xiWLUT0+bSXLvdE7TYgS5gCOSqSyXyTSSXiQ@mail.gmail.com