Consider an Autotools project that eventually wants to install some Guile
modules in the system. The canonical way to do this seems to add
GUILE_SITE_DIR to
https://www.gnu.org/software/guile/manual/html_node/Autoconf-Macros.html in
configure.ac and to use the variables GUILE_SITE and GUILE_SITE_CCACHE in
Makefile.am to determine the install targets.

However, this doesn't honor --prefix settings; in particular, make
distcheck fails for non-root users; see here for a general discussion:
https://www.gnu.org/software/automake/manual/html_node/Hard_002dCoded-Install-Paths.html

So in order to make use of GUILE_SITE and GUILE_SITE_CCACHE, I think we
need Autoconf macros in guile.m4 that allow to specify the prefix (as in
the Python example on the cited page).

-- Marc

P.S.: There is one more issue with guile.m4: When it finds the program
guile-2.2, it only searches for guild-2.2, but not for guild. However, at
least on Ubuntu systems, there is only guild, but no guild-2.2.

P.P.S.: Of course, in the long run we need integration of Guile in Autoconf
like Python and Elisp are already integrated.