uclibc++: gcc 5.2 build failure

[wongsyrone]

arm-openwrt-linux-muslgnueabi-g++ -fno-builtin -nostdinc++ -DGCC_HASCLASSVISIBILITY -I./include -MT src/fstream.o -MD -MP -MF src/.fstream.o.dep -Wall -Wno-trigraphs -pedantic -Os -pipe -march=armv7-a -mtune=cortex-a9 -mfpu=vfpv3-d16 -fno-caller-saves -fhonour-copts -Wno-error=unused-but-set-variable -mfloat-abi=soft -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -fpic -ansi -Os -fPIC -c src/fstream.cpp -o src/fstream.o

In file included from ./include/istream:24:0,
                 from ./include/fstream:28,
                 from src/fstream.cpp:22:
./include/ostream:298:50: error: default argument for template parameter for class enclosing 'class std::basic_ostream<charT, traits>::sentry'
   class _UCXXEXPORT basic_ostream<charT,traits>::sentry
                                                  ^
In file included from ./include/fstream:28:0,
                 from src/fstream.cpp:22:
./include/istream:343:107: error: default argument for template parameter for class enclosing 'class std::basic_istream<charT, traits>::sentry'
  template <class charT,class traits = char_traits<charT> > class _UCXXEXPORT basic_istream<charT,traits>::sentry {
                                                                                                           ^

[wongsyrone]

temporary use libstdcpp (GNU C++ Standard Library) if using gcc 5.1

[wongsyrone]

Fixed by upstream