build: allow disabling PPP support at build time

Pass --enable-ppp=no and pppd headers won't be required, and the
pppd plugin won't be built.  Here's to you, Gentoo!

configure.ac

@@ -359,9 +359,16 @@ else
 fi
 AC_SUBST(DBUS_SYS_DIR)
 
-# PPPD
-AC_CHECK_HEADERS(pppd/pppd.h,,
-  AC_MSG_ERROR(couldn't find pppd.h. pppd development headers are required.))
+# pppd
+AC_ARG_ENABLE(ppp, AC_HELP_STRING([--enable-ppp], [enable PPP/PPPoE support]),
+                     [enable_ppp=${enableval}], [enable_ppp=yes])
+if (test "${enable_ppp}" = "yes"); then
+	AC_CHECK_HEADERS(pppd/pppd.h,,
+		AC_MSG_ERROR(couldn't find pppd.h. pppd development headers are required.))
+
+	AC_DEFINE(WITH_PPP, 1, [Define if you have PPP support])
+fi
+AM_CONDITIONAL(WITH_PPP, test "${enable_ppp}" = "yes")
 
 AC_ARG_WITH([pppd-plugin-dir], AS_HELP_STRING([--with-pppd-plugin-dir=DIR], [path to the pppd plugins directory]))
 
@@ -372,6 +379,7 @@ else
 fi
 AC_SUBST(PPPD_PLUGIN_DIR)
 
+
 # dhclient support
 AC_ARG_WITH([dhclient], AS_HELP_STRING([--with-dhclient=yes|no|path], [Enable dhclient 4.x support]))
 # If a full path is given, use that and do not test if it works or not.
@@ -659,6 +667,12 @@ else
 	echo WiMAX support: no
 fi
 
+if test "${enable_ppp}" = "yes"; then
+	echo PPP support: yes
+else
+	echo PPP support: no
+fi
+
 echo
 echo Building documentation: ${with_docs}
 echo Building tests: ${with_tests}

src/ppp-manager/Makefile.am

@@ -33,6 +33,8 @@ libppp_manager_la_LIBADD = \
 	$(DBUS_LIBS) \
 	$(GLIB_LIBS)
 
+if WITH_PPP
+
 pppd_plugindir = $(PPPD_PLUGIN_DIR)
 pppd_plugin_LTLIBRARIES = nm-pppd-plugin.la
 
@@ -52,6 +54,8 @@ nm_pppd_plugin_la_LIBADD = \
 	$(DBUS_LIBS) \
 	$(GLIB_LIBS)
 
+endif
+
 BUILT_SOURCES = nm-ppp-manager-glue.h
 
 CLEANFILES = $(BUILT_SOURCES)

src/ppp-manager/nm-ppp-manager.c

@@ -958,6 +958,15 @@ nm_ppp_manager_start (NMPPPManager *manager,
 
 	priv = NM_PPP_MANAGER_GET_PRIVATE (manager);
 
+#if !WITH_PPP
+	/* PPP support disabled */
+	g_set_error_literal (err,
+	                     NM_PPP_MANAGER_ERROR,
+	                     NM_PPP_MANAGER_ERROR_UNKOWN,
+	                     "PPP support is not enabled.");
+	return FALSE;
+#endif
+
 	priv->pid = 0;
 
 	/* Make sure /dev/ppp exists (bgo #533064) */