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/lib-math/CMakeLists.txt

@@ -0,0 +1,31 @@
+#[[
+A library of mathematical utilities and manipulation of samples
+]]#
+
+set( SOURCES
+   Dither.cpp
+   Dither.h
+   FFT.cpp
+   FFT.h
+   InterpolateAudio.cpp
+   InterpolateAudio.h
+   Matrix.cpp
+   Matrix.h
+   RealFFTf.cpp
+   RealFFTf.h
+   SampleCount.cpp
+   SampleCount.h
+   SampleFormat.cpp
+   SampleFormat.h
+   Spectrum.cpp
+   Spectrum.h
+   float_cast.h
+)
+set( LIBRARIES
+   lib-preferences-interface
+   PRIVATE
+   wxBase
+)
+audacity_library( lib-math "${SOURCES}" "${LIBRARIES}"
+   "" ""
+)