docs/Makefile: use the curl.1 in the cvssource dir (again)

With the new docs build in the source dir, it harder to build it
separately (the *.d wildcard no longer works) but we also build the docs
more often now so doing it within the website as well is less important.

docs/Makefile

@@ -276,10 +276,7 @@ optionsall.html: _optionsall.html optionsall.gen
 optionsall.gen: $(DOCROOT)/options-in-versions alloptions.pl
 	./alloptions.pl $< > $@
 
-curl.1: ../cvssource/docs/cmdline-opts/*.d
-	(cd ../cvssource/docs/cmdline-opts && perl gen.pl mainpage *.d) > curl.1
-
-mandump.gen: curl.1
+mandump.gen: $(SRCROOT)/docs/curl.1
 	 $(MAN2HTML) < $< > mandump.gen
 
 mk-ca-bundle.html: _mk-ca-bundle.html $(MAINPARTS) mkdump.gen $(ROOT)/manpage.t