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><span title='2020-07-20 01:48:26 +0200'>2020-07-20</span></td><td><a href='/guix/commit/gnu/packages/time.scm?id=626bc647c06fff82d3d23ce1363f5b42977b49d8'>gnu: python-pendulum: Update to 2.1.1.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-07-12 23:08:15 +0200'>2020-07-12</span></td><td><a href='/guix/commit/gnu/packages/time.scm?id=b0e7b6992f3f845e83cfbca4d700b51dba50b4d5'>gnu: Remove ".git" from "https://github/…/….git".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-05-13 09:30:26 +0300'>2020-05-13</span></td><td><a href='/guix/commit/gnu/packages/time.scm?id=3c986a7dc268756c097212de958f046b29b76c1b'>mailmap: Update entries for Nikita.</a><span class='msg-avail'>...</span></td><td>nikita</td></tr>
<tr><td><span title='2020-04-08 13:00:50 +0200'>2020-04-08</span></td><td><a href='/guix/commit/gnu/packages/time.scm?id=27783023993f9272ce422868d14529159c4a5218'>Merge branch 'master' into core-updates</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-03-30 14:49:50 -0400'>2020-03-30</span></td><td><a href='/guix/commit/gnu/packages/time.scm?id=0ab3a8c24445c634c8696717361dcd9358f7a35a'>gnu: Add python-ciso8601.</a><span class='msg-avail'>...</span></td><td>Lars-Dominik Braun</td></tr>
<tr><td><span title='2020-03-21 22:16:02 +0100'>2020-03-21</span></td><td><a href='/guix/commit/gnu/packages/time.scm?id=d19b14c8349ce8cacb62619ab68953265daeeca7'>Merge branch 'master' into core-updates</a></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-03-20 22:41:59 +0100'>2020-03-20</span></td><td><a href='/guix/commit/gnu/packages/time.scm?id=e5023bfd2a0a51f84c5f9c59fda724db59c97cc8'>gnu: python-pendulum: Update to 2.1.0.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-03-20 22:41:58 +0100'>2020-03-20</span></td><td><a href='/guix/commit/gnu/packages/time.scm?id=463e98357bda9ce33aa3cc149418108b25bd1c16'>gnu: python-pytzdata: Update to 2019.3.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>