diff --git a/CMakeLists.txt b/CMakeLists.txt
index c20ea3a..be16680 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -61,6 +61,9 @@ add_library(xtl INTERFACE)
 target_include_directories(xtl INTERFACE $<BUILD_INTERFACE:${XTL_INCLUDE_DIR}>
                                          $<INSTALL_INTERFACE:include>)
 
+# xtl requires C++14 support!
+target_compile_features(xtl INTERFACE cxx_std_14)
+
 
 OPTION(BUILD_TESTS "xtl test suite" OFF)
 OPTION(DOWNLOAD_GTEST "build gtest from downloaded sources" OFF)
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index c2de281..0b20bc5 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -12,6 +12,7 @@ if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
     project(xtl-test)
 
     find_package(xtl REQUIRED CONFIG)
+    find_package(nlohmann_json QUIET CONFIG)
     set(XTL_INCLUDE_DIR ${xtl_INCLUDE_DIRS})
 endif ()
 
@@ -22,6 +23,10 @@ include(CheckCXXCompilerFlag)
 
 string(TOUPPER "${CMAKE_BUILD_TYPE}" U_CMAKE_BUILD_TYPE)
 
+if(nlohmann_json_FOUND)
+  add_definitions(-DHAVE_NLOHMANN_JSON)
+endif()
+
 if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Intel")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wunused-parameter -Wextra -Wreorder -Wconversion -Wsign-conversion")
     #set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wunused-parameter -Wextra -Wreorder -Wconversion")
@@ -30,13 +35,6 @@ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU"
     if (HAS_MARCH_NATIVE)
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -march=native")
     endif()
-
-    CHECK_CXX_COMPILER_FLAG("-std=c++14" HAS_CPP14_FLAG)
-    if (HAS_CPP14_FLAG)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
-    else()
-        message(FATAL_ERROR "Unsupported compiler -- xtl requires C++14 support!")
-    endif()
 endif()
 
 if(MSVC)
@@ -110,7 +108,17 @@ add_executable(${XTL_TARGET} ${XTL_TESTS} ${XTL_HEADERS})
 if(DOWNLOAD_GTEST OR GTEST_SRC_DIR)
     add_dependencies(${XTL_TARGET} gtest_main)
 endif()
-target_link_libraries(${XTL_TARGET} xtl nlohmann_json ${GTEST_BOTH_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT})
+target_link_libraries(${XTL_TARGET} xtl ${GTEST_BOTH_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT})
+
+if(nlohmann_json_FOUND)
+  # Version up to 3.1.2 export the target `nlohmann_json`
+  if(TARGET nlohmann_json)
+    target_link_libraries(${XTL_TARGET} nlohmann_json)
+  # Newer versions export the namespaced target `nlohmann_json::nlohmann_json`
+  elseif(TARGET nlohmann_json::nlohmann_json)
+    target_link_libraries(${XTL_TARGET} nlohmann_json::nlohmann_json)
+  endif()
+endif()
 
 add_custom_target(xtest COMMAND test_xtl DEPENDS ${XTL_TARGET})
 
diff --git a/test/test_xoptional.cpp b/test/test_xoptional.cpp
index 8100453..2ac1857 100644
--- a/test/test_xoptional.cpp
+++ b/test/test_xoptional.cpp
@@ -15,7 +15,11 @@
 
 #include "xtl/xany.hpp"
 #include "xtl/xoptional.hpp"
+
+#ifdef HAVE_NLOHMANN_JSON
 #include "xtl/xjson.hpp"
+#endif
+
 #include "xtl/xoptional_sequence.hpp"
 
 namespace xtl
@@ -276,6 +280,7 @@ namespace xtl
         EXPECT_EQ(res.has_value(), o.has_value());
     }
 
+#ifdef HAVE_NLOHMANN_JSON
     TEST(xoptional, json)
     {
         xoptional<double> m1 = missing<double>();
@@ -287,4 +292,5 @@ namespace xtl
         nlohmann::json j2 = m2;
         EXPECT_EQ(j2.get<xoptional<double>>(), 3.0);
     }
+#endif
 }