Add an EDM4HEP_WITH_JSON cmake option to be explicit

CMakeLists.txt

@@ -12,6 +12,11 @@ SET( ${PROJECT_NAME}_VERSION  "${${PROJECT_NAME}_VERSION_MAJOR}.${${PROJECT_NAME
 find_package(podio 1.1 REQUIRED HINTS $ENV{PODIO})
 find_package(ROOT REQUIRED COMPONENTS RIO Tree Physics)
 
+option(EDM4HEP_WITH_JSON "Whether or not to build EDM4HEP with JSON support (and edm4hep2json)" ON)
+if (EDM4HEP_WITH_JSON)
+ find_package(nlohmann_json 3.10.5 REQUIRED)
+endif()
+
 include(cmake/Key4hepConfig.cmake)
 
 #--- Define basic build settings -----------------------------------------------
@@ -63,7 +68,6 @@ install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/LICENSE
               ${CMAKE_CURRENT_SOURCE_DIR}/NOTICE
               DESTINATION ${CMAKE_INSTALL_DOCDIR})
 
-find_package(nlohmann_json 3.10.5)
 
 add_subdirectory(edm4hep)
 add_subdirectory(utils)

cmake/EDM4HEPConfig.cmake.in

@@ -14,7 +14,7 @@ set_and_check(EDM4HEP_DATA_DIR "@PACKAGE_CMAKE_INSTALL_DATADIR@/edm4hep")
 include(CMakeFindDependencyMacro)
 find_dependency(podio REQUIRED)
 
-set(EDM4HEP_WITH_JSON @nlohmann_json_FOUND@)
+set(EDM4HEP_WITH_JSON @EDM4HEP_WITH_JSON@)
 if (EDM4HEP_WITH_JSON)
   find_dependency(nlohmann_json 3.10.5 REQUIRED)
 endif()

edm4hep/CMakeLists.txt

@@ -19,7 +19,7 @@ add_library(EDM4HEP::edm4hep ALIAS edm4hep)
 file(GLOB_RECURSE top_headers ${PROJECT_SOURCE_DIR}/include/*.h)
 target_sources(edm4hep PUBLIC FILE_SET headers TYPE HEADERS BASE_DIRS ${PROJECT_SOURCE_DIR}/include FILES ${top_headers})
 
-if (nlohmann_json_FOUND)
+if (EDM4HEP_WITH_JSON)
   target_compile_definitions(edm4hep PUBLIC PODIO_JSON_OUTPUT)
   target_link_libraries(edm4hep PUBLIC nlohmann_json::nlohmann_json)
 endif()

tools/CMakeLists.txt

@@ -1,6 +1,6 @@
 list( APPEND CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}" )
 
-if (nlohmann_json_FOUND)
+if (EDM4HEP_WITH_JSON)
   add_executable(edm4hep2json src/edm4hep2json.cxx)
 
   set(EDM4HEP_SET_RPATH TRUE)