only use -ldl if available (netbsd compilation fix)

fixes #416

Makefile

@@ -29,7 +29,6 @@ GENH = src/version.h
 
 CFLAGS  += -Wall -O0 -g -std=c99 -D_GNU_SOURCE -pipe
 NO_AS_NEEDED = -Wl,--no-as-needed
-LIBDL   = -ldl
 LDFLAGS = -fPIC $(NO_AS_NEEDED) $(LIBDL) $(PTHREAD)
 INC     = 
 PIC     = -fPIC

configure

@@ -234,6 +234,11 @@ fi
 echo "$LD_SONAME_FLAG"
 echo "LD_SET_SONAME = -Wl,$LD_SONAME_FLAG," >> config.mak
 
+if check_link "checking whether we can use -ldl" "-ldl" \
+"int main(){return 0;}" ; then
+echo "LIBDL = -ldl" >> config.mak
+fi
+
 if check_link "checking whether we can use -lpthread" "-lpthread" \
 "int main(){return 0;}" ; then
 echo "PTHREAD = -lpthread" >> config.mak