Install docs via Gentoo mechanisms instead of the build system.

--- abcmidi/Makefile.in
+++ abcmidi/Makefile.in
@@ -143,13 +143,6 @@
 	$(INSTALL) -d $(DESTDIR)$(bindir)
 	$(INSTALL) -m 755 ${binaries} $(DESTDIR)$(bindir)
 
-	# install documentation
-	$(INSTALL) -d $(DESTDIR)${docdir}
-	$(INSTALL)  -m 644 doc/*.txt $(DESTDIR)$(docdir)
-	$(INSTALL)  -m 644 doc/AUTHORS $(DESTDIR)$(docdir)
-	$(INSTALL)  -m 644 doc/CHANGES $(DESTDIR)$(docdir)
-	$(INSTALL)  -m 644 VERSION $(DESTDIR)$(docdir)
-
 	# install manpages
 	$(INSTALL)  -d $(DESTDIR)${mandir}
 	$(INSTALL)  -m 644 doc/*.1 $(DESTDIR)$(mandir)