Fix backtrace detection with CMake 2.x.

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>

kmtrace/ConfigureChecks.cmake

@@ -4,8 +4,8 @@
 if ( CMAKE_MAJOR_VERSION LESS "3" )
   check_symbol_exists( backtrace "execinfo.h" HAVE_BACKTRACE )
   if ( HAVE_BACKTRACE )
-    set ( Backtrace_LIBRARY "" )
-    set ( Backtrace_HEADER "execinfo.h" )
+    set ( Backtrace_LIBRARY "" CACHE STRING "Library providing backtrace(3), empty for default set of libraries" )
+    set ( Backtrace_HEADER "execinfo.h" CACHE STRING "Header providing backtrace(3) facility" )
   endif ()
 else ()
   find_package( Backtrace )