[core][build] A collection of small detailed fixes and cleanups

[ethouris]

Important changes:

1. The configure script exposed CMAKE_CXX_COMPILER cmake variable only if --with-compiler-type option was provided; that should be done also if --with-compiler-prefix was added.
2. Added the codespell script so that you can run it manually before checkin.
3. The atomic field initialization in the app is removed; this is initialized with false anyway, but some gcc compilers choke on this (those with not-full-support of C++11).
4. SRTO_SENDER option for groups is simply ignored. This option is set internally in the application, so groups could not be really tested in this state.