Disable doc building the hard way - should rather be a config option
like libgpg-error's --disable-doc

diff -ur gpgmepp.org/Makefile.am gpgmepp/Makefile.am
--- gpgmepp.org/Makefile.am	2016-11-16 13:20:18.000000000 +0100
+++ gpgmepp/Makefile.am	2017-11-20 15:34:49.086731000 +0100
@@ -33,7 +33,7 @@
 tests =
 endif
 
-SUBDIRS = src ${tests} doc lang
+SUBDIRS = src ${tests} lang
 
 # Fix the version of the spec file.
 dist-hook: gen-ChangeLog