Without this patch, the 'make install' attempts to 'install' a directory, which causes an error.

diff --git a/c++/src/build-system/Makefile.in.top b/c++/src/build-system/Makefile.in.top
index 1abe44f..d6000b3 100644
--- a/c++/src/build-system/Makefile.in.top
+++ b/c++/src/build-system/Makefile.in.top
@@ -51,7 +51,7 @@ install-toolkit:
 	    done
 	cd $(includedir0) && find * -name CVS -prune -o -print |\
             cpio -pd $(pincludedir)
-	$(INSTALL) -m 644 $(incdir)/* $(pincludedir)
+	cp -R $(incdir)/* $(pincludedir)
 ## set up appropriate build and status directories somewhere under $(libdir)?
 
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<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
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<div class='path'>path: <a href='/guix/log/?id=83eac82c5ebaf014146f52e12cdd1729b61fc545'>root</a>/<a href='/guix/log/etc?id=83eac82c5ebaf014146f52e12cdd1729b61fc545'>etc</a>/<a href='/guix/log/etc/system-tests.scm?id=83eac82c5ebaf014146f52e12cdd1729b61fc545'>system-tests.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/system-tests.scm?id=83eac82c5ebaf014146f52e12cdd1729b61fc545&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-08-09 15:16:06 +0200'>2022-08-09</span></td><td><a href='/guix/commit/etc/system-tests.scm?id=5bce4c82422de6beb3ce6120ba1592be898c2b72'>build-system: Add 'channel-build-system'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/ci.scm (channel-build-system, channel-source-&gt;package): Remove.
* gnu/packages/package-management.scm (channel-source-&gt;package): New
procedure, moved from (gnu ci).
* guix/build-system/channel.scm: New file, with code moved from (gnu ci).
* doc/guix.texi (Build Systems): Document it.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-07-10 23:57:14 +0200'>2022-07-10</span></td><td><a href='/guix/commit/etc/system-tests.scm?id=9fdc4b6c283c5aa5cf10205d87fb2c58b829b9d0'>monads: Add 'mparameterize'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* etc/system-tests.scm (mparameterize): Move to...
* guix/monads.scm (mparameterize): ... here.
* tests/monads.scm ("mparameterize"): New test.
* .dir-locals.el (c-mode): Add it.
</span></span></td><td>Ludovic Courtès</td></tr>