Haiku's (https://www.haiku-os.org/) g++ needs -fpermissive to find memcpy(), because of the conflicting prototype will throw error. However mkconfig doesn't test if -fpermissive needs to be passed during tests. Passing it manually using CFLAGS kind of defeats the purpose of using mkconfig.
clang++ refuses to run the tests because the extension of the test is .c, but the code is actually C++ due to including multiple C++ headers on top of the test.
I have uploaded version 4.54 with fixes for C++ and support for the -fpermissive flag. Please try it out.
Now works when CC is set to gcc, like it should be...