find_path does not work when cross-compiling

CMakeLists.txt

@@ -10,19 +10,12 @@ option(BUILD_EXAMPLES   "Build examples" ON)
 if(NOT TARGET json-hpp)
     set(NLOHMANN_JSON_DIR "" CACHE STRING "path to json.hpp")
 
-    # find nlohmann's json.hpp
+    if(NOT EXISTS ${NLOHMANN_JSON_DIR}/nlohmann/json.hpp)
-    find_path(JSON_HPP nlohmann/json.hpp
-        PATHS
-            ${NLOHMANN_JSON_DIR}
-            ${CMAKE_BINARY_DIR}/${NLOHMANN_JSON_DIR}) # in case it is a relative path
-
-    # get the full, real path
-    get_filename_component(NLOHMANN_JSON_REALPATH ${JSON_HPP} REALPATH)
-
-    if(NOT EXISTS ${NLOHMANN_JSON_REALPATH}/nlohmann/json.hpp)
         message(FATAL_ERROR "please set NLOHMANN_JSON_DIR to a path in which NLohmann's json.hpp can be found. Looking for nlohmann/json.hpp in '${NLOHMANN_JSON_REALPATH}")
     endif()
 
+    get_filename_component(NLOHMANN_JSON_REALPATH ${NLOHMANN_JSON_DIR} REALPATH)
+
     # create an interface-library for simple cmake-linking
     add_library(json-hpp INTERFACE)
     target_include_directories(json-hpp