build: Add missing build-aux files to EXTRA_DIST

* Makefile.am: Likewise.
build: Link canvas and stopwatch against C math library

* nclua/Makefile.am (canvas_la_LDFLAGS): Likewise.
* nclua/event/Makefile.am (stopwatch_la_LDFLAGS): Likewise.
* configure.ac (LIBM): New variable, math library linker flags; defined
by Libtool's macro LT_LIB_M.