Various CMakeLists adjustments

Various things that bothered me while working on other PRs / downstream packaging.

(I CBA to split them up further)

• Try to get smbclient flags via pkg-config
  Non-x64 CI is unhappy with completely switching to it, so the previous code is still used as a fallback.

• Simplify Gettext module check
  It's a Find* package, so find it. Mark it required instead of the custom handling.

• Split CMAKE_CXX_FLAGS assignment into correct variables/calls
  CMAKE_CXX_STANDARD for -std=c++11, add_compile_options for the other settings.