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_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/2a919372655d64a5ed848b84a09b54d685aec446

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