build-sys: add verbosity to python-lxml detection in ./configure

Be verbose when checking if Python module lxml is available.  Also warn that
Python support will be disabled when the lxml module is not present.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=80005

Tested:
- Without python-lxml package installed:
  $ ./configure
  checking for python extension module directory... ${exec_prefix}/lib64/python2.7/site-packages
  checking for python lxml module... no
  configure: WARNING: *** python support requires python-xml module installed

- With python-lxml package installed:
  $ ./configure
  checking for python extension module directory... ${exec_prefix}/lib64/python2.7/site-packages
  checking for python lxml module... yes
  checking for PYTHON_DEVEL... yes
  ...
          Python:                  yes
          Python Headers:          yes

1 files changed, 7 insertions, 2 deletions

diff --git a/configure.ac b/configure.ac
index 1391d033b4..1aff72d3b1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -216,8 +216,13 @@ AC_ARG_WITH([python],
 have_lxml=no
 AS_IF([test "x$with_python" != "xno"], [
         AM_PATH_PYTHON(,, [:])
-        AS_IF(["$PYTHON" -c 'import lxml' 2>/dev/null], [have_lxml=yes], [have_lxml=no])
-        AS_IF([test "$PYTHON" != : -a $have_lxml = yes], [have_python=yes])
+        AS_IF([test "x$PYTHON" != "x:"], [
+                AC_MSG_CHECKING([for python lxml module])
+                AS_IF(["$PYTHON" -c 'import lxml' 2>/dev/null], [have_lxml=yes])
+                AC_MSG_RESULT([$have_lxml])
+                AS_IF([test "x$have_lxml" = "xyes"], [have_python=yes],
+                      [AC_MSG_WARN([*** python support requires python-lxml module installed])])
+        ])
 ])
 AM_CONDITIONAL([HAVE_PYTHON], [test "x$have_python" = "xyes"])
 AS_IF([test "x$PYTHON_BINARY" = "x"],