Find sphinx-build in configure, and explicitly invoke under python2 or python3 as required

findsphinx.patch

diff --git a/xapian-bindings/configure.ac b/xapian-bindings/configure.ac
index 89b3f8c..c9c316d 100644
--- a/xapian-bindings/configure.ac
+++ b/xapian-bindings/configure.ac
@@ -483,6 +483,15 @@ if test x$USE_MAINTAINER_MODE = xyes; then
   test -z "$PYTHON" && AC_MSG_ERROR([python is required in maintainer mode])
 fi
 
+dnl If we're building documentation for either python version,
+dnl we need sphinx-build
+case $enable_documentation$with_python$with_python3 in
+yes*yes*)
+  AC_PATH_PROG(SPHINX_BUILD, sphinx-build, [])
+  test -z "$SPHINX_BUILD" && AC_MSG_ERROR([sphinx-build is required to build python documentation])
+  ;;
+esac
+
 if test no != "$with_php" ; then
   dnl See comment for AC_PATH_PROGS(SWIG, ...).
   AC_PATH_PROGS(PHP_CONFIG, ["${PHP_CONFIG-php-config}"], [])
diff --git a/xapian-bindings/python/Makefile.am b/xapian-bindings/python/Makefile.am
index a838cd8..e4cfe78 100644
--- a/xapian-bindings/python/Makefile.am
+++ b/xapian-bindings/python/Makefile.am
@@ -158,7 +158,7 @@ dist-hook: $(sphinxdocs)
 	cp -R -p docs/html $(distdir)/docs
 
 $(sphinxdocs): xapian/__init__.py docs/conf.py $(srcdir)/docs/*.rst $(dist_exampledata_DATA)
-	PYTHONPATH=..:$$PYTHONPATH sphinx-build -b html -d doctrees -c docs $(srcdir)/docs docs/html
+	PYTHONPATH=..:$$PYTHONPATH $(PYTHON2) $(SPHINX_BUILD) -b html -d doctrees -c docs $(srcdir)/docs docs/html
 	@echo  
 	@echo "Build finished. The HTML pages are in docs/html."
 endif
diff --git a/xapian-bindings/python3/Makefile.am b/xapian-bindings/python3/Makefile.am
index c2aa3f0..75ba433 100755
--- a/xapian-bindings/python3/Makefile.am
+++ b/xapian-bindings/python3/Makefile.am
@@ -162,10 +162,8 @@ dist-hook: $(sphinxdocs)
 	test -d $(distdir)/docs || mkdir $(distdir)/docs
 	cp -R -p docs/html $(distdir)/docs
 
-SPHINX_BUILD = /usr/share/sphinx/scripts/python3/sphinx-build
-
 $(sphinxdocs): xapian/__init__.py docs/conf.py $(srcdir)/docs/*.rst $(dist_exampledata_DATA)
-	PYTHONPATH=..:$$PYTHONPATH $(SPHINX_BUILD) -b html -d doctrees -c docs $(srcdir)/docs docs/html
+	PYTHONPATH=..:$$PYTHONPATH $(PYTHON3) $(SPHINX_BUILD) -b html -d doctrees -c docs $(srcdir)/docs docs/html
 	@echo  
 	@echo "Build finished. The HTML pages are in docs/html."
 endif