After cloning the libcxx repository:

gettextize -f
git checkout .    # gettextize does some unnecessary scribbling, get rid of it
libtoolize
aclocal
autoheader
automake --add-missing
sysconftoolize
autoreconf
./configure
make

Note: the build might attempt to refresh effective_tld_names.dat from
publicsuffix.org, and try to download it.

This is only needed if checking out directly from the repository, which does
not store autogenerated files.

Then see libcxx documentation for complete build instructions.