pgsql: Cleanup NAN code in float.h, too.

Cleanup NAN code in float.h, too.

In the same spirit as 3bf905692, assume that all compilers we still
support provide the NAN macro, and get rid of workarounds for that.

The C standard allows implementations to omit NAN if the underlying
float arithmetic lacks quiet (non-signaling) NaNs. However, we've
required that feature for years: the workarounds only supported
lack of the macro, not lack of the functionality. I put in a
compile-time #error if there's no macro, just for clarity.

Also fix up the copies of these functions in ecpglib, and leave
a breadcrumb for the next hacker who touches them.

History of the hacks being removed here can be found in commits
1bc2d544b, 4d17a2146, cec8394b5.

Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/1952095.1759764279@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/14ad0d7bf2b8d5f26061df7cbdf18cfffcdcb225

Modified Files
--------------
src/include/utils/float.h | 40 ++++++++++++++------------------------
src/interfaces/ecpg/ecpglib/data.c | 19 ++++--------------
2 files changed, 19 insertions(+), 40 deletions(-)