build: revive the ocaml package tests

* autogen.sh: Generalize the ocaml-package-existence test.
Remove the git-related part of the old test.

autogen.sh

@@ -42,6 +42,32 @@ if [ ! -z "$BUILDDIR" ]; then
     CONFIGUREDIR=..
 fi
 
+# Ensure that an ocaml package is present for build-from sources.
+# This is *not* for anything that is required at configure-time
+# when configure is run from a distribution tarball.  From those,
+# nothing ocaml-related is required.
+# ocamlfind cannot detect the presence of -devel packages directly,
+# so if $pkg ends in -devel, first check for the base package, and
+# if that's found, check for the existence of $base.cmxa in the
+# resulting directory.
+require_ocaml_pkg()
+{
+  pkg=$1
+  case $pkg in
+    *-devel)
+      local base=${pkg%%-devel}
+      local dir=$(ocamlfind query "$base") || return 1
+      test -f "$dir/$base.cmxa" || return 1
+      ;;
+    *) ocamlfind query "$pkg" > /dev/null 2>&1 || return 1;;
+  esac
+  return 0
+}
+
+{ require_ocaml_pkg xml-light && require_ocaml_pkg xml-light-devel; } \
+  || { echo "you must have ocaml, ocamlfind, ocaml-xml-light" \
+         "and ocaml-xml-light-devel" >&2; exit 1; }
+
 # If no arguments were specified and configure has run before, use the previous
 # arguments
 if [ $# == 0 -a -x ./config.status ]; then