New library for math...

... note the swap of target_link_libraries lines in src/CMakeLists.txt,
needed to build at least on macOS, becuase FFT.h must be looked up first in
lib-math, not in lib-src/twolame

Also making a dependency cycle of SampleFormat and Dither!  But we will tolerate
that within one small library.

libraries/CMakeLists.txt

@@ -11,6 +11,7 @@ set( LIBRARIES
    lib-basic-ui
    lib-exceptions
    lib-preferences
+   lib-math
 )
 
 if ( ${_OPT}has_networking )