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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e71c76fcab8d00defe9a7a608facdd9663f7bcbf

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