configure: add --extra-cxxflags option

same usage as --extra-cflags

Change-Id: Iff2ed7b8ebb6e51610ee0851aeec08413367ab23
diff --git a/configure b/configure
index 0e01a2e..a40f3ab 100755
--- a/configure
+++ b/configure
@@ -723,6 +723,10 @@
         check_add_cflags ${extra_cflags} || \
         die "Requested extra CFLAGS '${extra_cflags}' not supported by compiler"
     fi
+    if [ -n "${extra_cxxflags}" ]; then
+        check_add_cxxflags ${extra_cxxflags} || \
+        die "Requested extra CXXFLAGS '${extra_cxxflags}' not supported by compiler"
+    fi
 }