printf-safe checks of invalid long double values

[Pádraig Brady]

I noticed that vfprintf() was replaced on my glibc-2.20 system,
which is surprising and a lot of code to be duplicating.

This gets pulled in because:

 $ grep gl_cv_func_printf.*=no config.log
 gl_cv_func_printf_infinite_long_double=no

 $ grep -A5 "checking.*infinite 'long double' arguments" config.log
 configure:22775: checking whether printf supports infinite 'long double' 
arguments
 configure:23082: gcc -std=gnu99 -o conftest -march=native -g -O2   conftest.c  
>&5
 configure:23082: $? = 0
 configure:23082: ./conftest
 configure:23082: $? = 120
 configure: program exited with status 120

Digging in a little with the attached conftest.c, all the failures
are due to the checking of pseudo infinity values.  Now I've
no knowledge yet of floating point formats, but I'm surprised
that the replacement seems to give the same non NaN values that
the replaced system vfprintf outputs in this case:

$ printf '\x00\x00\x00\x00\x00\x00\x00\00\xFF\xFF\x00\x00\x00\x00\x00\x00' | 
src/od -t fL
0000000                      -0e+4912

Questions I have at present:
  Are these checks backed up by corresponding replacement code?
  Are these checks correct?
  Why has glibc not been updated in the 7 years since the checks were added?

cheers,
Pádraig.

p.s. one can avoid these checks by unsetting gl_printf_safe

conftest.c
Description: Text document