configure: also look for pygobject-3.0 runtime dependency.

If we decide to look for Python dependencies, we should also look for
pygobject as it provides the GI binding to Python.
I'm still not sure if we should look for these runtime dependencies at
configure time. But for the time being, if we continue doing so, let's
do it right.

configure.ac

@@ -2237,18 +2237,18 @@ AM_CONDITIONAL(WITH_PDBGEN, test "x$with_pdbgen" = xyes)
 
 GOBJECT_INTROSPECTION_REQUIRE(introspection_required_version)
 
-##################
-# Check for python
-##################
+#######################################
+# Check for python runtime dependencies
+#######################################
 
 # By default, we want packagers to install Python plug-ins to get the
 # optimum experience. --with-python=yes will check for a Python 3
-# interpreter and fails without.
-# --with-python=force will install the plug-ins even though the Python
-# interpreter is not found at build time. It is up to the packager to
-# ensure one is available at run time. This can be useful in particular
-# when cross-compiling since anyway the interpreter is not useful at
-# build time.
+# interpreter and PyGObject, and fails without.
+# --with-python=force will install the plug-ins even though the
+# dependencies are not found at build time. It is up to the packager to
+# ensure they are available at run time. This can be useful in
+# particular when cross-compiling since anyway the interpreter is not
+# useful at build time.
 # --with-python=no won't install Python plug-ins. It is discouraged.
 AC_ARG_WITH(python,
             [  --with-python=no|yes|force install Python 3 plug-ins (default=yes)], ,
@@ -2264,31 +2264,40 @@ WARNING: you disabled the installation of core Python plug-ins. This is
          Just make sure that a Python python3_required_version or newer
          interpreter is available at run-time."
 elif test "x$with_python" = xforce; then
-  AM_PATH_PYTHON3(python3_required_version,
-                  with_python="yes",
-                  with_python="yes (Warning: Python python3_required_version or newer not found)")
-  if test "x$with_python" != "xyes"; then
+  with_python="yes"
+  warning_python=
+  AM_PATH_PYTHON3(python3_required_version, ,
+                  warning_python="Python python3_required_version or newer not found
+")
+  PKG_CHECK_MODULES(PYGOBJECT, pygobject-3.0, ,
+                    warning_python="PyGObject 3.0 not found
+$warning_python")
+  if test "x$warning_python" != "x"; then
     warning_python="
-WARNING: Python python3_required_version or newer was not found.
-         Python plug-ins will be installed anyway but you should make
-         sure that a compatible Python interpreter is available at
-         installation, otherwise installed plug-ins won't be usable."
+WARNING: ${warning_python}         Python plug-ins will be installed anyway but you should make
+         sure that a compatible Python interpreter and PyGObject are
+         available at installation, otherwise installed plug-ins won't
+         be usable."
+    with_python="yes (see warning below)"
   fi
-else # with_python=yes
-  # Mandatory Python check.
-  m4_define([pycheck_error], [
+else
+  with_python="yes"
+  error_python=
+  AM_PATH_PYTHON3(python3_required_version,,
+                  error_python="- Python python3_required_version or newer not found
+")
+  PKG_CHECK_MODULES(PYGOBJECT, pygobject-3.0, ,
+                    error_python="- PyGObject 3.0 not found
+$error_python")
+  if test "x$error_python" != "x"; then
     required_deps="$required_deps
-    - $1
-        *** Please install $2.
+    $error_python        *** Please install Python 3 and PyGObject 3.
         *** Note that you may install the Python plug-ins even if you have
         *** no Python interpreter at build-time by passing the configure
         *** option --with-python=force.
-        *** Just make sure that a Python python3_required_version or newer
-        *** interpreter is available at run-time."
-    with_python="no"])
-
-  AM_PATH_PYTHON3(python3_required_version,,
-                 [pycheck_error([Python 3 (python3_required_version or newer)], [it])])
+        *** Just make sure that the Python dependencies are available at run-time."
+    with_python="no"
+  fi
 fi
 
 if test "x$with_python" = "xyes"; then