FTBFS with GCC 11

nut is FTBFS'ing when compiled with GCC 11 due to:

https://launchpadlibrarian.net/552666656/buildlog_ubuntu-impish-amd64.nut_2.7.4-13ubuntu4_BUILDING.txt.gz

libtool: compile: g++ -DHAVE_CONFIG_H -I. -I../include -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -ffile-prefix-map=/<<PKGBUILDDIR>>=. -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security -c nutclient.cpp -fPIC -DPIC -o .libs/nutclient.o
In file included from nutclient.cpp:20:
nutclient.h:135:86: error: ISO C++17 does not allow dynamic exception specifications
  135 | virtual void authenticate(const std::string& user, const std::string& passwd)throw(NutException)=0;
      | ^~~~~
nutclient.h:141:30: error: ISO C++17 does not allow dynamic exception specifications
  141 | virtual void logout()throw(NutException)=0;
      | ^~~~~
nutclient.h:154:58: error: ISO C++17 does not allow dynamic exception specifications
  154 | virtual Device getDevice(const std::string& name)throw(NutException);
      | ^~~~~
nutclient.h:159:46: error: ISO C++17 does not allow dynamic exception specifications
  159 | virtual std::set<Device> getDevices()throw(NutException);
      | ^~~~~
...

This is happening because GCC 11 defaults to C++17 which doesn't allow dynamic exception specifications.

Upstream has fixed the bug:

https://github.com/networkupstools/nut/commit/fab323320d5b955ed034b2eea390a9bbb549e8e5

I'm preparing an MP and will post it soon.