configure.ac: Add check for python3 libs

They are needed since commit
c1e04e4069
("scripts/unocommands.py: Switch to python3"),
so add a corresponding check to configure.ac.

Change-Id: If0f1d2b474c3edb3fc63dc54292acd54f096b822
Reviewed-on: https://gerrit.libreoffice.org/81473
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Michael Weghorn <m.weghorn@posteo.de>

configure.ac

@@ -50,6 +50,17 @@ AS_IF([test `uname -s` = Linux],
        AS_IF([test -z "$SETCAP"],
        [AC_MSG_ERROR([The setcap command is needed. It might be in a package called libcap-progs.])]))
 
+# check for python modules
+for MODULE in lxml polib; do
+    AC_MSG_CHECKING([for ${MODULE} for python3])
+    if /usr/bin/env python3 -c "import ${MODULE}" 2> /dev/null ; then
+        AC_MSG_RESULT([yes])
+    else
+        AC_MSG_RESULT([no])
+        AC_MSG_ERROR([${MODULE} for python3 is needed. It might be in a package called python3-${MODULE}.])
+    fi
+done
+
 # Declare options
 AC_ARG_ENABLE([debug],
               AS_HELP_STRING([--enable-debug],