configure.ac: Use LIBS rather than LDFLAGS to add -ldl to dladdr check

ec8ebff "Check for dladdr()" erroneously uses LDFLAGS rather than LIBS to add
-ldl to the dladdr check.

Replace the workaround in 39a4cc4 of explicitly checking in libdl, with a more
correct approach of using LIBS.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>