configure: trust pkg-config when it's used for zlib

configure: trust pkg-config when it's used for zlib

The library flags retrieved from pkg-config were later thrown out and
harded-coded, which negates the whole reason to use pkg-config.
Also, previously, the assumption was made that --libs-only-l and
--libs-only-L are the full decomposition of --libs, which is untrue and
would not allow linking against a static zlib. The new approach is
better in that it uses --libs, although only if --libs-only-l returns
nothing.

Bug: https://curl.se/mail/lib-2023-08/0081.html
Reported-by: Randall
Closes #11778