Allow build without libnm installed

libnm is not actually required, but rather used only to determine the
value of NM_VPN_SERVICE_DIR. So it's fine to build with NM support but
without the lib. Since we already provide a default for the parameter
print a warning instead of a fatal message in case the value couldn't
be determined.

(cherry picked from commit 1256297108)

CMakeLists.txt

@@ -1343,12 +1343,18 @@ endif()
 
 if( WITH_NETWORK_MANAGER_BACKEND )
   pkg_search_module( LIBNM libnm )
-  if( NOT LIBNM_FOUND )
-    tde_message_fatal( "libnm was requested but it was not found on your system." )
-  endif( NOT LIBNM_FOUND )
-  pkg_get_variable( NM_VPN_SERVICE_DIR libnm vpnservicedir )
+  if( LIBNM_FOUND )
+    pkg_get_variable( NM_VPN_SERVICE_DIR libnm vpnservicedir )
+  endif( )
   if( "${NM_VPN_SERVICE_DIR}" STREQUAL "" )
     set( NM_VPN_SERVICE_DIR "/usr/lib/NetworkManager/VPN" )
+    message( WARNING "NetworkManager support was requested, but the value for NM_VPN_SERVICE_DIR "
+                     "couldn't be determined (either because NetworkManger or libnm's pkg-config "
+                     "files are not installed or they don't declare it explicitly). "
+                     "Falling back to the default value:\n"
+                     "     ${NM_VPN_SERVICE_DIR}\n"
+                     "If the value doesn't suite your needs consider setting it manually by "
+                     "passing \"-DNM_VPN_SERVICE_DIR=/path/to/your/NetworkManager/VPN\" to cmake." )
   endif( "${NM_VPN_SERVICE_DIR}" STREQUAL "" )
 endif( WITH_NETWORK_MANAGER_BACKEND )