libupnpp: needs gcc >= 4.9

libupnpp: needs gcc >= 4.9

After bumping libupnpp from version 0.15.1 to 0.15.2 compilation fails
for toolchains using GCC 4.7:

```
libupnpp/control/discovery.cxx: In constructor 'UPnPClient::UPnPDeviceDirectory::UPnPDeviceDirectory(time_t)':
libupnpp/control/discovery.cxx:338:5: error: 'yield' is not a member of 'std::this_thread'
```

Before version 0.15.2, libupnpp used sched_yield() which was replaced by
`std::this_thread::yield()` in the new version.

Looking at the `<thread>` header file of the PowerPC toolchain [1] from
the autobuilder shows that `yield()` is only enabled if
`_GLIBCXX_USE_SCHED_YIELD` is defined:

```
    /// yield
    inline void
    yield() noexcept
    { __gthread_yield(); }
```

Note, that `__gthread_yield()` inlines to a call to `sched_yield()`:

```
static inline int
__gthread_yield (void)
{
  return __gthrw_(sched_yield) ();
}
```

This macro is only defined if GCC was built with `--enable-libstdcxx-time`,
which is obviously not the case for the affected toolchains. For GCC 4.7 and
below this option defaults to no, which means `sched_yield()` is not enabled
though it is be available.

The issue was discussed on stackoverflow [2]. In GCC 4.9.0 the issue has
been fixed, by automatically enabling `sched_yield()` on platforms that are
known to support them.

Therefore, we update the dependency to GCC 4.9.

Fixes:
http://autobuild.buildroot.net/results/506/50670d8119b4fbd26585d3d9bd646adb1d904dbe/
http://autobuild.buildroot.net/results/8b0/8b021648a073bde25dd6936ed1ea1e39977bf89b/
http://autobuild.buildroot.net/results/b6d/b6d7437e1cf8cf0b3726395401ac4ebd9af2833b/
http://autobuild.buildroot.net/results/0bf/0bf84f61a72a65a75c5fd0a787680f483cfead26/
.. and more.

[1] http://autobuild.buildroot.org/toolchains/tarballs/powerpc-ctng_e500v2-linux-gnuspe.tar.xz
[2] https://stackoverflow.com/a/12961816

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>