maint: merge build improvements from coreutils

maint: merge build improvements from coreutils

* configure.ac: Invoke gl_ASSERT_NO_GNULIB_POSIXCHECK.
(--enable-gcc-warnings): Change help message.
(gl_GCC_VERSION_IFELSE): New macro.
Do not omit -Wunused-macros for main code.
Adjust other -W options as per coreutils.
* lib/Makefile.am (AM_CFLAGS): Use GNULIB_WARN_CFLAGS, not WARN_CFLAGS.
* unlzw.c (REGISTERS, REG1, REG2, ..., REG16): Remove.
All uses removed.  These provoked -Wunused-macros warnings.
This sort of fiddling with registers hasn't been needed for years.