improve 'clean_all' target (still not perfect but should be good enough)

CMakeLists.txt

@@ -278,11 +278,12 @@ set_property(
 
 if(NOT TARGET clean_all)
   add_custom_target(clean_all
+    # useful only after 'cmake ."
+    # cowardly not doing rm -r ${CMAKE_CURRENT_BINARY_DIR} ${CMAKE_BINARY_DIR}
     COMMAND ${CMAKE_BUILD_TOOL} clean
-    COMMAND find . -name "*.o" -o -name "*.d" -o -regex "./libgencmp-.*" | xargs rm
-    COMMAND find . -name build -type dir | xargs rm -r
+    COMMAND find . -name "*.o" -o -name "*.d" -o -regex "./libgencmp-.*" | xargs rm || true
     COMMAND find . -path ./libsecutils/Makefile | xargs -I % ${CMAKE_BUILD_TOOL} -C libsecutils clean_all || true
-    COMMAND find . -path ./cmpossl/Makefile | xargs -I % ${CMAKE_BUILD_TOOL} -C cmpossl clean_all || true
+    COMMAND find . -path     ./cmpossl/Makefile | xargs -I % ${CMAKE_BUILD_TOOL} -C cmpossl clean_all || true
     COMMAND rm CMakeCache.txt
     # after the following, cannot call this target again:
     COMMAND find . ( -name "*.cmake" -o -name Makefile )
@@ -291,6 +292,7 @@ if(NOT TARGET clean_all)
     COMMAND find . -name CMakeFiles
             -not -path ./libsecutils/* -not -path ./cmpossl/*
             | xargs rm -r
+    COMMAND find . -name build -type dir | xargs rm -r || true
     VERBATIM
  )
  endif()