work around Coq bug #18434

In Coq 8.18, coqdoc's --external flag had its arguments flipped (see
https://github.com/coq/coq/issues/18434). To work around this, we
provide both

	--external https://math-comp.github.io/htmldoc/ mathcomp

and

	--external mathcomp https://math-comp.github.io/htmldoc/

One will be understood by Coq 8.17 and earlier, the other works for
Coq 8.18.

This hack can be reverted when upgrading to a version of Coq in which
the regression has been fixed (presumably 8.19).

Makefile

@@ -59,7 +59,7 @@ mangle-namesCoqProject: commonCoqProject
 	  -print | scripts/module-toc-order.py >> $(COQ_PROJ)
 
 $(COQ_MAKEFILE): $(COQ_PROJ) scripts/Makefile.coq.patch
-	@coq_makefile -f $< -o $@ COQDOCEXTRAFLAGS = "--parse-comments --external https://math-comp.github.io/htmldoc/ mathcomp"
+	@coq_makefile -f $< -o $@ COQDOCEXTRAFLAGS = "--parse-comments --external https://math-comp.github.io/htmldoc/ mathcomp --external mathcomp https://math-comp.github.io/htmldoc/"
 	@# Patch HTML target to switch out color.
 	@# Also include Makefile.coqdocjs for 'htmlpretty' documentation target.
 	@patch -s < scripts/Makefile.coq.patch