src/mathutils.c: improve build with -Wdeclaration-after-statement
src/spectral/mfcc.c: improve build with -Wdeclaration-after-statement
src/io/source_sndfile.c: improve build with -Wdeclaration-after-statement
src/spectral/filterbank_mel.c: improve build with -Wdeclaration-after-statement
examples/utils.c: remove old comment
wscript: do not pass -Wextra to msvc
src/aubio_priv.h: use ifdef for macros that can be undefined