Fix AIX compilation issue with NAN and INFINITY

[ayappanec]

We have recently ported R to AIX .
https://www.ibm.com/developerworks/aix/library/aix-toolbox/alpha.html#R
When trying to install data.table, there is a compilation issue with NAN and INFINITY.

fread.c:79:28: error: initializer element is not constant
static const double NAND = (double)NAN;
^
fread.c:80:28: error: initializer element is not constant
static const double INFD = (double)INFINITY;
^
With gcc -E option , we see this

static const unsigned int _SINFINITY = 0x7f800000;
static const unsigned int _SQNAN = 0x7fc00000;

static const double NAND = (double)
79 "fread.c" 3 4
(*((float )(&_SQNAN)))
79 "fread.c"
;
static const double INFD = (double)
80 "fread.c" 3 4
(((float *)(&_SINFINITY)))
80 "fread.c"

So they don't seem to qualify as constant literals hence the compilation issue.
I have tested the fix in Linux as well.

[codecov]

Codecov Report

Merging #3043 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3043      +/-   ##
==========================================
+ Coverage   90.87%   90.87%   +<.01%     
==========================================
  Files          61       61              
  Lines       11734    11740       +6     
==========================================
+ Hits        10663    10669       +6     
  Misses       1071     1071

Impacted Files	Coverage Δ
src/fread.c	97.96% <100%> (ø)	⬆️

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[jangorecki]

Hi, thanks for catching that, appreciate your PR.
Would be even better if you could:

• add entry in NEWS
• put comment next to init() declaration referring issue/PR number

Also note that we have Contributing wiki which describes how to make PR.
You have good timing, we are preparing 1.11.6 release. Next 1.12.0 will probably takes some time, so we should include this for 1.11.6.

[ayappanec]

Thanks. I have updated the PR as per your info.

[jangorecki]

LGTM