Fixed message

CMakeLists.txt

@@ -48,8 +48,11 @@ if (OJL_BIMAP_BUILD_TESTS)
     check_cxx_compiler_flag(-std=c++20 CXX_20_SUPPORT)
 
     if (NOT CXX_20_SUPPORT)
-        message(FATAL_ERROR "The C++ compiler does not support the -std=c++20 flag")
-        message(FATAL_ERROR "Use a compiler that supports C++ 20, like clang++ >= 10")
+        message(FATAL_ERROR "\
+            The C++ compiler does not support the -std=c++20 flag\n\
+            Use a compiler that supports C++ 20, like clang++ >= 10\
+            ")
+        message(FATAL_ERROR 
     endif()