Mirror OAN_HOST_BACKENDS option at OAN root for standalone builds

The host-backend define was only injected by the parent OALS CMakeLists.
That meant standalone OAN builds (e.g. CI on this repo) would compile
with OAN_HOST_BACKENDS undefined regardless of the -D flag — and then
NetworkMapper.cpp's #else branch would reference firmware externs
(_now_ms, _now_us, _fetch_iface_meta), failing to link.

Mirror OALS's option block here: opt-in OFF by default, forced ON on
APPLE (no other transport works), translated to a compile definition.

Author: jonathan-reichardt

CMakeLists.txt

@@ -7,5 +7,18 @@ if(NO_THREADS)
     add_compile_definitions(NO_THREADS)
 endif (NO_THREADS)
 
+# OAN_HOST_BACKENDS: switches in the host-side transport seam (SimTransport,
+# RawLinuxTransport, RawMacTransport). The define is normally injected by
+# the parent OALS build; mirror the logic here so OAN is self-buildable
+# (e.g. for CI of OAN-only changes). Forced ON on macOS — there is no
+# other working transport on Apple at the moment.
+option(OAN_HOST_BACKENDS "Build host dev transports + RT shim" OFF)
+if(APPLE)
+    set(OAN_HOST_BACKENDS ON CACHE BOOL "Build host dev transports + RT shim" FORCE)
+endif()
+if(OAN_HOST_BACKENDS)
+    add_compile_definitions(OAN_HOST_BACKENDS)
+endif()
+
 add_subdirectory(common)
 add_subdirectory(netutils)