config FEATURE compile-time customization of CH backoff

Refs cesnet/netopeer2#1264
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8d0bdfd..bf44bc8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -98,6 +98,7 @@
 set(MAX_PSPOLL_THREAD_COUNT 6 CACHE STRING "Maximum number of threads that could simultaneously access a ps_poll structure")
 set(TIMEOUT_STEP 100 CACHE STRING "Number of microseconds tasks are repeated until timeout elapses")
 set(YANG_MODULE_DIR "${CMAKE_INSTALL_PREFIX}/share/yang/modules" CACHE STRING "Directory with common YANG modules")
+set(CALL_HOME_BACKOFF_WAIT 2 CACHE STRING "Number of seconds to wait between Call Home connection attempts")
 
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 # sources