build: don't define _GNU_SOURCE manually

build: don't define _GNU_SOURCE manually

Now that we're using gnulib in earnest, any manual definition
would provoke a redefinition warning.
* fish/fish.c (_GNU_SOURCE): Don't define.
* fish/destpaths.c (_GNU_SOURCE): Likewise.
* src/guestfs.c (_GNU_SOURCE): Likewise.
* bootstrap (modules): Add asprintf, strchrnul, strerror, strndup
and vasprintf.
* fish/fish.c (main): Set argv[0] to sanitized program_name, so
functions like getopt_long that use argv[0] use the clean name.