pgsql: configure: check for dlsym instead of dlopen.

configure: check for dlsym instead of dlopen.

When building with sanitizers the sanitizer library provides dlopen, but not
dlsym(), making configure think that -ldl isn't needed. Just checking for
dlsym() ought to suffice, hard to see dlsym() being provided without dlopen()
also being provided.

Backpatch to all branches, for the same reasons as 46ab07ffda9.

Reviewed-By: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20220323173537.ll7klrglnp4gn2um@alap3.anarazel.de
Backpatch: 10-

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e52e9bd57f33da1c4a915e24d0a24759a23cb5fd

Modified Files
--------------
configure    | 27 +++++++++++++++------------
configure.ac |  5 ++++-
2 files changed, 19 insertions(+), 13 deletions(-)