Add CMake support

[basicmaster]

Thanks for your PR - this is a great improvement compared to the current Makefile.

I took a look into the commits and identified some minor issues, so I would like to suggest the following fixes:

• ATM both executables being build are the CLI version dablin.cpp - the sources lists should rather look e.g. like the following:

list(APPEND dablin_sources
    dabplus_decoder.cpp
    eti_source.cpp
    eti_player.cpp
    dab_decoder.cpp
    fic_decoder.cpp
    sdl_output.cpp
    tools.cpp
    )

list(APPEND dablin_cli_sources
    dablin.cpp
    )

list(APPEND dablin_gtk_sources
    pad_decoder.cpp
    dablin_gtk.cpp
    )

• the Fraunhofer AAC lib is fdk-aac, not fdk-aac-dabplus - which is our OpenDigitalradio AAC encoder ;-)
• FAAD2 should not be REQUIRED as someone may choose to use fdk-aac instead (besides, the REQUIRED check did not work for me, as CMake did not abort when I previously removed FAAD2 for testing - although the later if check worked of course)
• the fec lib can be included via ${FEC_LIBRARIES} (instead of ${FEC}) and the lone fec therefore removed
• IMO the dablin_gtk target should only be created/compiled/installed when gtkmm ist installed (GTKMM therefore should not be REQUIRED, so that CLI only people do not have to install gtkmm)