more configure tweaks

[mattdowle]

More follow up to #3964
This time not Solaris but something odd on CRAN's macOS.

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From Kurt :

For the prev submission, Prof Ripley found the following:

This version does not install for me on macOS:

pkg-config did not detect zlib is installed. Please install it:
* deb: zlib1g-dev (Debian, Ubuntu, ...)
* rpm: zlib-devel (Fedora, EPEL, ...)
* brew: zlib (OSX)
ERROR: configuration failed for package ‘data.table’

macOS does not have pkg-config, nor does it supply .pc files for its
system software (SU has packaged some, but not zlib nor for current
macOS).  And a pkg-config-only test is not portable for other platforms.

Can you pls fix accordingly?

Best
-k

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From Matt :

Thanks, Kurt and Prof Ripley.

I don't assume that pkg-config is installed, but it appears then that it is installed on that macOS box (as R-exts note 25 says) but, oddly, pkg-config is claiming zlib is not installed when we know it is (since current CRAN error with recent 1.12.4 on macOS is runtime error long after compilation, which stands to reason since zlib is required by R itself). This all works fine on Travis macOS so it is not macOS per se.

However, I can see a fix I can make for this situation; i.e., pkg-config being installed but not returning an accurate result. I'll fix and resubmit.

I'm hoping this means the previous attempt works now on Solaris then? I can see Rrof Ripley added 'checkbashisms' to R-exts note 24 as I suggested (so he received that email), but I didn't hear back from him after that 2nd Solaris attempt. (I didn't receive the reply you've quoted from him about macOS.)

Best, Matt