build: new configure option --enable-gcc-warnings

build: new configure option --enable-gcc-warnings

This has a similar meaning as in other GNU applications
such as coreutils and Emacs.
* NEWS: Document it.
* .gitignore: Remove redundant build-aux.
Remove gnu, since gnu/Makefile.am is now in git.
Add gnu/.gitignore, gnu/charset.alias, gnu/*.h, gnu/*/ to cover
autogenerated files.
* bootstrap.conf (gnulib_mk): Remove.
* configure.ac: Add support for --enable-gcc-warnings,
taken from coreutils and simplified.
* gnu/Makefile.am: New file.  Formerly this was autogenerated,
but the autogenerated file has been renamed to gnulib.mk,
its usual name when bootstrapping from gnulib.
This way, AM_CFLAGS can incorporate warning options.
* gnulib.modules: Add manywarnings.
* lib/Makefile.am, src/Makefile.am (AM_CFLAGS): New macro,
incorporating warning options.
* lib/attr-xattr.in.h (ENOATTR): New macro, if not already defined.
* src/buffer.c (magic): Don't rely on incomplete initializers.
* src/common.h (report_difference): Add printf format attribute.
* src/system.c (sys_exec_command, sys_exec_info_script)
(sys_exec_checkpoint_script):
* src/tar.c (update_argv):
Add casts to char * to pacify GCC warnings about using string
literals in a char * context.
* src/xattrs.c, src/xattrs.h (xattrs_clear_setup):
Declare parameters as (void), not ().
* src/xheader.c (xheader_format_name): Initialize pptr to null,
to pacify GCC.  Remove unnecessary test of nptr versus null.