[cmake] require c++11 as the minimum standard

[lrusak]

Introduced in #45 lambdas require a minimum of c++11 standard in order to build. This is needed because clang seems to default to c++98 by default. see, https://clang.llvm.org/cxx_status.html

cmake -DCMAKE_CXX_STANDARD=98 ..
make

results in failure

/home/lukas/Documents/git/platform/src/util/StringUtils.cpp: In static member function ‘static std::string& StringUtils::TrimLeft(std::string&)’:
/home/lukas/Documents/git/platform/src/util/StringUtils.cpp:456:99: warning: lambda expressions only available with ‘-std=c++11’ or ‘-std=gnu++11’
  456 |   str.erase(str.begin(), ::find_if(str.begin(), str.end(), [](char s) { return isspace_c(s) == 0; }));
      |                                                                                                   ^

This PR fixes this issue

[Hedda]

@malard @opdenkamp any updates on this?