build: Add missing tests and gnulib modules suggested by autoscan. * build-aux/bootstrap_gnulib.conf: Add float and stddef header modules. * libgnu/Makefile.am: Automatically re-generated when module list changed. * configure.ac: Add function check for isascii, modf, pow, sqrt. Add function check for chown, ftruncate, mmap, munmap when building GUI. * libinterp/interpfcn/toplev.cc: Add gnulib:: qualifier to malloc call. * liboctave/util/oct-shlib.cc: Add gnulib:: qualifier to strerror call.