cmake: Remove libunwind-generic as hard dependency

when using alternate unwind implementations like LLVM libunwind
this library is not provided yet the libunwind features are fully
implemented in main libunwind, making this hard dependency assumes
a particular libunwind implementation, this patch makes it optional
which makes the builds to work with llvm libunwind

Signed-off-by: Khem Raj <raj.khem@gmail.com>

CMakeLists.txt

@@ -1365,7 +1365,7 @@ elseif(UNIX AND NOT APPLE AND NOT ANDROID AND NOT RISCOS AND NOT HAIKU)
       if(HAVE_LIBUNWIND_H)
         # We've already found the header, so REQUIRE the lib to be present
         pkg_search_module(UNWIND REQUIRED libunwind)
-        pkg_search_module(UNWIND_GENERIC REQUIRED libunwind-generic)
+        pkg_search_module(UNWIND_GENERIC libunwind-generic)
         list(APPEND EXTRA_LIBS ${UNWIND_LIBRARIES} ${UNWIND_GENERIC_LIBRARIES})
       endif()
     endif()