Correct the usage of LDFLAGS

For https://bugzilla.gnome.org/show_bug.cgi?id=761252

It is no longer necessary to save system LDFLAGS when checking
lib specific LDFLAGS.
diff --git a/configure.ac b/configure.ac
index 03302d6..5ffa01f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -159,7 +159,7 @@
   if test "$withval" != "no" -a "$withval" != "yes"; then
     RDL_DIR=$withval
     CPPFLAGS="${CPPFLAGS} -I$withval/include"
-    LIBS="${LIBS} -L$withval/lib"
+    LDFLAGS="${LDFLAGS} -L$withval/lib"
   fi
 ])
 AC_ARG_WITH(regexps,
@@ -195,7 +195,7 @@
   if test "$withval" != "no" -a "$withval" != "yes"; then
     Z_DIR=$withval
     CPPFLAGS="${CPPFLAGS} -I$withval/include"
-    LIBS="${LIBS} -L$withval/lib"
+    LDFLAGS="${LDFLAGS} -L$withval/lib"
   fi
 ])
 AC_ARG_WITH(lzma,
@@ -203,7 +203,7 @@
   if test "$withval" != "no" -a "$withval" != "yes"; then
     LZMA_DIR=$withval
     CPPFLAGS="${CPPFLAGS} -I$withval/include"
-    LIBS="${LIBS} -L$withval/lib"
+    LDFLAGS="${LDFLAGS} -L$withval/lib"
   fi
 ])
 AC_ARG_WITH(coverage,