ensure -ldl is passed only once

Author: picnixz

configure

@@ -2989,9 +2989,10 @@ AC_HEADER_MAJOR
 AC_CHECK_HEADERS([execinfo.h link.h dlfcn.h], [
   AC_CHECK_FUNCS([backtrace dladdr1], [
     # dladdr1 requires -ldl
-    AS_VAR_APPEND([LDFLAGS], [" -ldl"])
+    ac_cv_require_ldl=yes
   ])
 ])
+AS_VAR_IF([ac_cv_require_ldl], [yes], [AS_VAR_APPEND([LDFLAGS], [" -ldl"])])
 
 # bluetooth/bluetooth.h has been known to not compile with -std=c99.
 # http://permalink.gmane.org/gmane.linux.bluez.kernel/22294