chore: fix building with njson

Signed-off-by: Andrea Terzolo <andrea.terzolo@polito.it>

CMakeLists.txt

@@ -148,16 +148,7 @@ include(falcosecurity-libs)
 include(jq)
 
 # nlohmann-json
-set(NJSON_SRC "${PROJECT_BINARY_DIR}/njson-prefix/src/njson")
+include(njson)
-message(STATUS "Using bundled nlohmann-json in '${NJSON_SRC}'")
-set(NJSON_INCLUDE "${NJSON_SRC}/single_include")
-ExternalProject_Add(
-  njson
-  URL "https://github.com/nlohmann/json/archive/v3.3.0.tar.gz"
-  URL_HASH "SHA256=2fd1d207b4669a7843296c41d3b6ac5b23d00dec48dba507ba051d14564aa801"
-  CONFIGURE_COMMAND ""
-  BUILD_COMMAND ""
-  INSTALL_COMMAND "")
 
 # b64
 include(b64)

cmake/modules/njson.cmake

@@ -0,0 +1,34 @@
+#
+# Copyright (C) 2023 The Falco Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+# the License. You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+# specific language governing permissions and limitations under the License.
+#
+
+#
+# nlohmann-json
+#
+if(NJSON_INCLUDE)
+    # Adding the custom target we can use it with `add_dependencies()`
+    if(NOT TARGET njson)
+        add_custom_target(njson)
+    endif()
+else()
+    # We always use the bundled version
+    set(NJSON_SRC "${PROJECT_BINARY_DIR}/njson-prefix/src/njson")
+    set(NJSON_INCLUDE "${NJSON_SRC}/single_include")
+    ExternalProject_Add(
+        njson
+        URL "https://github.com/nlohmann/json/archive/v3.3.0.tar.gz"
+        URL_HASH "SHA256=2fd1d207b4669a7843296c41d3b6ac5b23d00dec48dba507ba051d14564aa801"
+        CONFIGURE_COMMAND ""
+        BUILD_COMMAND ""
+        INSTALL_COMMAND "")
+    message(STATUS "Using bundled nlohmann-json in '${NJSON_SRC}'")
+endif()

cmake/modules/yaml-cpp.cmake

@@ -19,6 +19,7 @@ if(NOT USE_BUNDLED_DEPS)
   else()
     message(FATAL_ERROR "Couldn't find system yamlcpp")
   endif()
+  add_custom_target(yamlcpp)
 else()
   set(YAMLCPP_SRC "${PROJECT_BINARY_DIR}/yamlcpp-prefix/src/yamlcpp")
   message(STATUS "Using bundled yaml-cpp in '${YAMLCPP_SRC}'")

userspace/engine/CMakeLists.txt

@@ -29,9 +29,7 @@ set(FALCO_ENGINE_SOURCE_FILES
 
 add_library(falco_engine STATIC ${FALCO_ENGINE_SOURCE_FILES})
 
-if(USE_BUNDLED_DEPS)
+add_dependencies(falco_engine yamlcpp njson)
-  add_dependencies(falco_engine yamlcpp njson)
-endif()
 
 if(MINIMAL_BUILD)
   target_include_directories(