Why does gnulib generate alloca.h, sys/types.h, unistd.h on GNU/Linux?

[Paul Smith]

I've switched over to using a few gnulib modules in GNU make.

I've just noticed that whenever I run configure, even though I'm on a
GNU/Linux system with GNU libc 2.27 and a fairly modern GCC 7.3, the
configure script insists on generating local copies of standard header
files:

  lib/alloca.h
  lib/stdlib.h
  lib/sys/types.h
  lib/unistd.h

Why do local copies of these files get generated on my (ostensibly
modern and compliant) system?

I attempted to decipher the configure scripting around these headers
but gave up and went to bed.  I did discover that for alloca.h, if we
see the __GNUC__ preprocessor value set we apparently ALWAYS choose the
local implementation and never the default implementation:

  #if defined __GNUC__ || defined _AIX || defined _MSC_VER
          Need own alloca
  #endif

That seems very strange to me!

The other headers I didn't make much headway on.

Any info would be interesting to me, thanks!