configure.ac: fix linking when using compiler sanitizers

The GCC address sanitizer overrides dlopen and dlclose, so the configure
test does not detect libdl as a needed dependency for linking. It is
still needed though, as dlsym is not exported by the sanitizer. The
result is that linking fails in the end.

Fix this by checking for dlsym instead of dlopen.

This can be reproduced by configuring the build with:
CFLAGS="-fsanitize=address -fsanitize=undefined"
LDFLAGS="-fsanitize=address -fsanitize=undefined"

Signed-off-by: Arnaud Vrac <rawoul@gmail.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>