The Makefile would still force the use of and linking against uClibc++
even though libstdc++ may have been chosen, which would result in the
package depending on libstdcpp ($(CXX_DEPENDS) but we would still be
missing an libuClibc++ library depdency.

Fix this by looking at CONFIG_USE_UCLIBCXX to adjust the configure
script variables.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>