The file $(top_srcdir)/gtk-doc.make doesn't exist, so don't include it.

--- a/doc/Makefile.in	2007-04-08 20:17:25.000000000 +0200
+++ b/doc/Makefile.in	2014-09-06 23:19:41.413397662 +0200
@@ -240,9 +240,6 @@
 distclean-generic clean-generic maintainer-clean-generic clean \
 mostlyclean distclean maintainer-clean
 
-
-include $(top_srcdir)/gtk-doc.make
-
 # Tell versions [3.59,3.63) of GNU make to not export all variables.
 # Otherwise a system limit (for SysV at least) may be exceeded.
 .NOEXPORT:
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<tr><td><span title='2020-08-29 16:24:17 +0200'>2020-08-29</span></td><td><a href='/guix/commit/build-aux/pre-inst-env.in?id=d7965b58d25c9c7648ba5682223f32d759fdffda'>build: Remove references to the 'nix-hash' program.</a><span class='msg-avail'>...<span class='msg-tooltip'>* configure.ac: Remove check for 'nix-hash'.
* tests/base32.scm (%nix-hash, %have-nix-hash?): Remove.
("sha256 &amp; bytevector-&gt;nix-base32-string"): Remove test.
* build-aux/pre-inst-env.in: Do not set 'NIX_HASH' environment
variable.
</span></span></td><td>Ludovic Courtès</td></tr>