Fix build with libunwind (fix up for ad1c78ab) (#759)

* cmake/FindUnwind.cmake used the wrong variable (caused by commit ad1c78ab)
* CMakeLists.txt seems to use wrong syntax for providing a compiler define
  of `WITH_UNWIND` (used in `arbor/util/unwind.cpp`)

CMakeLists.txt

@@ -281,7 +281,7 @@ endif()
 find_package(Unwind)
 if(Unwind_FOUND)
     target_link_libraries(arbor-private-deps INTERFACE Unwind::unwind)
-    target_compile_definitions(arbor-private-deps ARB_WITH_UNWIND)
+    target_compile_definitions(arbor-private-deps INTERFACE WITH_UNWIND)
 
     list(APPEND arbor_export_dependencies "Unwind")
 endif()

cmake/FindUnwind.cmake

@@ -34,7 +34,7 @@ if(NOT Unwind_FOUND)
         PATH_SUFFIXES lib64 lib
     )
 
-    find_library(_unwind_library_target unwind-${libunwind_arch}
+    find_library(_unwind_library_target unwind-${_libunwind_arch}
         HINTS ${Unwind_SEARCH_DIR}
         PATH_SUFFIXES lib64 lib
     )