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_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a77dddb42e0916f8c0962b758668d49e8fd81cc8

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