Use an update guile.m4; it seems different systems (Fedora, arch,
gentoo, ...) have guile installed under different names. Try to handle
all of those.
For this, we include the upstream guile.m4 and add support for
guile-snarf.
some distros -- notably Ubuntu 16.04 -- do not include
ax_cxx_compile_stdcxx_14.m4 in their autotools-archive package (since
the macro is too new).
This breaks the compilation since we need that macro to get the correct
c++14 flags. So, let's add them ourselves, so users don't have to shop
around for these macros themselves.
Since we depend on autoconf-archive now, we don't need the copies of
some of the m4 files, esp. since those break the autoconf-archive check
in configure.ac.
