fix compiler warning in stacktrace.c
    
        Prevents warning:
        > cc1: warning: command line option ‘-Wno-non-virtual-dtor’ is valid for C++/ObjC++ but not for C
    
      * added FWEELIN_CXXFLAGS to src/Makefile.am to avoid passing the -Wno-non-virtual-dtor flag to gcc
      * replaced c-cast in src/fweelin_midiio.cc from Jajcus/compiler_warnings patch with static_cast
      * reverted patch in src/stacktrace.c from Jajcus/compiler_warnings guarding unused my_getline()
          in favor of a dummy call becasue my_getline() was intended to be called from outside that guard