Disable libbacktrace support if the library is not present in the system.

Author: magomez

Source/cmake/OptionsWPE.cmake

@@ -419,7 +419,8 @@ endif ()
 if (USE_LIBBACKTRACE)
     find_package(LibBacktrace)
     if (NOT LIBBACKTRACE_FOUND)
-        message(FATAL_ERROR "libbacktrace is required for USE_LIBBACKTRACE")
+        message(WARNING "libbacktrace not found, disabling support")
+        SET_AND_EXPOSE_TO_BUILD(USE_LIBBACKTRACE OFF)
     endif ()
 endif ()