Do not blindly add libdl to extra libraries

On some systems dlopen() is available without libdl (illumos, solaris).
Sometimes libdl.so cannot be loaded by runtime linker, see
https://ghc.haskell.org/trac/ghc/ticket/8713

Closes #8

configure.ac

@@ -186,7 +186,7 @@ else
 fi
 
 # Avoid adding dl if absent or unneeded
-AC_CHECK_LIB(dl, dlopen, [EXTRA_LIBS="$EXTRA_LIBS dl"])
+AC_SEARCH_LIBS([dlopen], [dl], [EXTRA_LIBS="$EXTRA_LIBS $ac_lib"])
 
 # -{l,}pthread goo
 AC_CANONICAL_TARGET