Define SEASTAR_MODULE only for module interface file

The SEASTAR_MODULE macro should only be defined when compiling the module
interface file (seastar.cppm), not for the implementation .cc files.

When .cc files are compiled with SEASTAR_MODULE defined, headers expand
SEASTAR_MODULE_EXPORT_BEGIN to "export {", which is invalid outside a
module purview, causing compilation errors like:
  "export declaration can only be used within a module purview"

Now SEASTAR_MODULE is set only for seastar.cppm using set_source_files_properties,
while implementation files compile without it, making export macros expand to
nothing in regular translation units.

Co-authored-by: avikivity <[email protected]>

Author: Copilot

src/CMakeLists.txt

@@ -98,8 +98,11 @@ target_compile_definitions (seastar-module
     SEASTAR_API_LEVEL=${Seastar_API_LEVEL}
     SEASTAR_SCHEDULING_GROUPS_COUNT=${Seastar_SCHEDULING_GROUPS_COUNT}
   PRIVATE
-    SEASTAR_MODULE
     ${Seastar_PRIVATE_COMPILE_DEFINITIONS})
+# Define SEASTAR_MODULE only for the module interface file, not implementation files
+set_source_files_properties (seastar.cppm
+  PROPERTIES
+    COMPILE_DEFINITIONS "SEASTAR_MODULE")
 target_compile_options (seastar-module
   PUBLIC
     -U_FORTIFY_SOURCE)