Fix build on systems that really like gnu strerror_r

[mheyman]

Cross compiling to aarch64 from x64 caused issues using the arm-provided toolchain where there was no way to tell reproc that it was to use the posix strerror_r. The pull replaces the original code with a pattern pulled from curl that is indifferent to whether the signature of strerror_r is gnu-style or posix-style. This uses the _Generic keyword from C11.