Hygiene only when project is top-level

CHANGES.md

@@ -15,6 +15,7 @@
   like `__declspec(dllexport)`
 - Add target variants `CapnC::Runtime-Shared`, `CapnC::Runtime-Static` and
   `CapnC::Runtime-StaticExports`
+- Hygiene only runs when the project is top-level.
 
 ## 0.9.0
 

CMakeLists.txt

@@ -35,7 +35,8 @@ option(BUILD_SHARED_LIBS "Build shared libraries (.dll/.so) instead of static on
 # we try to enforce build hygiene.
 # You can override with either BUILD_HYGIENE=DISABLED or
 # BUILD_HYGIENE=ENABLED.
-if(NOT BUILD_HYGIENE STREQUAL DISABLED)
+# Hygiene is only done if the project is top-level.
+if(PROJECT_IS_TOP_LEVEL AND NOT BUILD_HYGIENE STREQUAL DISABLED)
     if(BUILD_HYGIENE STREQUAL ENABLED)
         set(find_program_args REQUIRED)
     else()