Using glibc headers with non-GCC compilers which define _Float32 etc.

[Florian Weimer <fweimer@redhat.com>]

We have a downstream bug that points out that if __GNUC__ is not
defined, glibc assumes that it's necessary to define _Float32 as a
typedef to float:

  glibc: _FloatN and _FloatNx problem if _GNUC__ is not defined
  <https://bugzilla.redhat.com/show_bug.cgi?id=2183212>

That does not work if the compiler already defines _Float32.

What should we do about this?  The suggestion to provide the definition
for __STDC_WANT_IEC_60559_TYPES_EXT__ requires additional glibc header
file changes, I think.  Other headers assume that _Float32 is available,
one way or another, and if __STDC_WANT_IEC_60559_TYPES_EXT__ is defined
and the compiler does not _Float32 (say Clang, or old GCC), then those
headers will break.

Thanks,
Florian