build: require python module libxml2

configure should warn if libxml2 is not found.
libxml2 is needed by glapi/gen.

Fixes error during build in src/mapi/glapi/gen:
ImportError: No module named libxml2

NOTE: This is a candidate for the 9.0 branch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31598
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

configure.ac

@@ -49,6 +49,7 @@ AM_PROG_CC_C_O
 AM_PROG_AS
 AC_CHECK_PROGS([MAKE], [gmake make])
 AC_CHECK_PROGS([PYTHON2], [python2 python])
+AX_PYTHON_MODULE([libxml2], [needed])
 AC_PROG_SED
 AC_PROG_MKDIR_P
 

m4/ax_python_module.m4

@@ -0,0 +1,49 @@
+# ===========================================================================
+#     http://www.gnu.org/software/autoconf-archive/ax_python_module.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_PYTHON_MODULE(modname[, fatal])
+#
+# DESCRIPTION
+#
+#   Checks for Python module.
+#
+#   If fatal is non-empty then absence of a module will trigger an error.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Andrew Collier
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 6
+
+AU_ALIAS([AC_PYTHON_MODULE], [AX_PYTHON_MODULE])
+AC_DEFUN([AX_PYTHON_MODULE],[
+    if test -z $PYTHON;
+    then
+        PYTHON="python"
+    fi
+    PYTHON_NAME=`basename $PYTHON`
+    AC_MSG_CHECKING($PYTHON_NAME module: $1)
+	$PYTHON -c "import $1" 2>/dev/null
+	if test $? -eq 0;
+	then
+		AC_MSG_RESULT(yes)
+		eval AS_TR_CPP(HAVE_PYMOD_$1)=yes
+	else
+		AC_MSG_RESULT(no)
+		eval AS_TR_CPP(HAVE_PYMOD_$1)=no
+		#
+		if test -n "$2"
+		then
+			AC_MSG_ERROR(failed to find required module $1)
+			exit 1
+		fi
+	fi
+])