Fix FTBFS when backtrace is not found.

If backtrace is not found - for example, with Musl C library,
then Backtrace_LIBRARY instead of empty value contains
Backtrace_LIBRARY-NOT_FOUND. This results in all libraries
listed in LINK_PRIVATE for tdecore-shared are ignored.

This resolves FTBFS on Dragora and issue TDE/tde-packaging-gentoo#277.

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

CMakeLists.txt

@@ -377,6 +377,9 @@ else ()
   find_package( Backtrace )
   if ( Backtrace_FOUND )
     set ( HAVE_BACKTRACE 1 )
+  else ()
+    set ( Backtrace_LIBRARY "" )
+    set ( Backtrace_HEADER "" )
   endif ()
 endif ()
 check_cxx_source_compiles( "#include <cxxabi.h>