* README-hacking: Suggest to use ./configure --quiet so that
any warnings are easily noticed.
* m4/gmp.m4 (cu_GMP): Warn if libgmp is not available.
* m4/jm-macros.m4 (coreutils_MACROS): Normalize the libcap warning.
* m4/xattr.m4 (gl_FUNC_XATTR): Warn if libattr is not available.