pgsql: oauth: Correct missing comma in Requires.private

[Jacob Champion]

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 <walt...@technowledgy.de>
Reviewed-by: Fabrízio de Royes Mello <fabriziome...@gmail.com>
Discussion: 
https://postgr.es/m/caoymi+k2z7rqj5xiwlut0+bsxlvde7tygs5gcosqsyxytss...@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a8f093234d78ff2bbcf6489ac213064aa584ad2b

Modified Files
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src/interfaces/libpq/Makefile | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)