CMake: rely on compile features to inject at least C++11

[SpaceIm]

• forcing -std=c++11 or using CXX_STANDARD is not recommended in a library, since it prevents users to compile the library with a more recent C++ standard.
• also remove hardcoded C++ standard from generated pkgconfig file

cxx_std_11 is supported since CMake 3.8, so it's fine since current min CMake version of fcl is 3.10

---

This change is