newlib: merge acconfig.h changes into newlib.hin

newlib: merge acconfig.h changes into newlib.hin

The acconfig.h header was used to run autoheader and then manually
sync the output into newlib.hin.  Based on how the files have fallen
out of sync (with newlib.hin having many more templates), this has
not been run in a long time, and attempts to do so now would break
newlib.hin.

Further, if you try to run autoheader now, it will automatically
replace _newlib_version.hin since it's the first entry in the call
to AC_CONFIG_HEADERS.

So let's throw away acconfig.h entirely.  It only had 2 slightly
better comments, and the rest were either worse, missing, or stale.
This has the side benefit of avoiding autoheader warning about the
deprecated use of acconfig.h since newer autoconf only wants macro
calls in configure.ac.