fwrite zlib

[mattdowle]

Closes #3939
Closes #3872

The configure scripts in other R packages seem to be copies of the Autoconf program, at about 5,000 lines long. Plus an associated short configure.ac I just can't wrap my head around. It's just so complicated with those layers.
So let's just try a short simple script. The comment at the top starts "let's keep this simple". If this works then to pass a flag through to the code could be a next step. But honesty, zlib is so standard, that I don't think we should invest effort and complexity in supporting no-zlib. If and when users start to complain they don't have zlib for some reason, then we can reconsider at that point.

Also removed z_const so it should work on MacOS now. I kept it as const rather than removing it because it seems safer to protect that input with const. If any of the C++ compilation problems arise that the zlib developers referred to then we can reconsider at that point. Keeping it as const resulting in this warning so I removed the const just as @philippechataignon thought in the first place :

gcc -std=gnu99 -I"/usr/share/R/include" -DNDEBUG    -fopenmp -fpic  -O0 -g -c fwrite.c -o fwrite.o
fwrite.c: In function ‘compressbuff’:
fwrite.c:569:19: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   stream->next_in = (const Bytef *)source; // don't use z_const anywhere; #3939

There was a discussion about gcc version in #3291. If that's true and we do need a certain version or later, then it could be checked in this configure script, with helpful advice output before the impending compile error.

[codecov]

Codecov Report

Merging #3951 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #3951   +/-   ##
======================================
  Coverage    99.4%   99.4%           
======================================
  Files          72      72           
  Lines       13642   13642           
======================================
  Hits        13561   13561           
  Misses         81      81

Impacted Files	Coverage Δ
src/fwrite.c	97.83% <100%> (ø)	⬆️

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[MichaelChirico]

What was wrong with const exactly?

I see this in my zlib.h:

/usr/include/zconf.h:234:#if defined(ZLIB_CONST) && !defined(z_const)
/usr/include/zconf.h:235:#  define z_const const
/usr/include/zconf.h-236-#else
/usr/include/zconf.h:237:#  define z_const
/usr/include/zconf.h-238-#endif

would it make sense to simply imitate this logic like

#ifndef zconst
#define z_const
#endif 

?

[mattdowle]

I put the error message I got in the comment at the top of this issue.
I don't understand what z_const is for. If we put those 3 lines back in, could you think of a comment to put alongside to explain why if z_const is defined, it should be used? And why const can't be used? master as of now is simple: don't use z_const.

[MichaelChirico]

Sorry I missed the comment, I had only seen the commit message.

I don't really know any details, mine was more a question of curiosity... perhaps @philippechataignon could comment based on motivation for using z_const in the first place?

[jangorecki]

Nice PR.
Yes I agree we could lets first see if it actually hits users before we will try to add much more complexity to handle that.