Fix include of nlohmann_json dependency

JSONParser/CMakeLists.txt

@@ -4,8 +4,8 @@ cmake_minimum_required( VERSION 3.1 )
 project( "JSONParser" VERSION 1.0.0 LANGUAGES CXX )
 
 find_package( GeoModelCore  REQUIRED )
-find_package( nlohmann_json REQUIRED )
-find_package(Eigen3 REQUIRED)
+find_package( nlohmann_json QUIET )
+find_package( Eigen3 REQUIRED )
 
 
 # Set default build options.
@@ -24,7 +24,12 @@ file( GLOB HEADERS JSONParser/*.h )
 add_library( JSONParser SHARED ${HEADERS} ${SOURCES} )
 set_property( TARGET JSONParser
    PROPERTY PUBLIC_HEADER ${HEADERS} )
-target_link_libraries( JSONParser PUBLIC nlohmann_json::nlohmann_json GeoModelCore::GeoModelKernel )
+if( NLOHMANN_JSON_FOUND )
+  target_link_libraries( JSONParser PUBLIC nlohmann_json::nlohmann_json GeoModelCore::GeoModelKernel )
+else()
+  message(STATUS "'nlohmann_json' not found by CMake!! Anyway, if you installed the single header file in a standard system include dir, I will be able to use it.")
+  target_link_libraries( JSONParser PUBLIC GeoModelCore::GeoModelKernel )
+endif()
 target_include_directories( JSONParser SYSTEM PUBLIC ${EIGEN3_INCLUDE_DIR} )
 target_include_directories( JSONParser PUBLIC
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
@@ -43,4 +48,3 @@ set(MYLIB_VERSION_PATCH 0)
 set(MYLIB_VERSION_STRING ${MYLIB_VERSION_MAJOR}.${MYLIB_VERSION_MINOR}.${MYLIB_VERSION_PATCH})
 
 set_target_properties(JSONParser PROPERTIES VERSION ${MYLIB_VERSION_STRING} SOVERSION ${MYLIB_VERSION_MAJOR})
-