Build failure on macOS 10.15

[Julien ÉLIE]

Hi Russ,
>> Suggested fix:
> 
> I've applied the relevant part of this to rra-c-util as well, with a minor
> tweak to the comment and to the configure output (because so far as I can
> tell Autoconf doesn't promise to always set $LD).

Oh, thanks!
I'm wondering whether RRA_PROG_LD_FLAG should not take 2 arguments into 
account ($1 for CFLAGS, and $2 for LDFLAGS).

Use case would be to improve the current chek for PIE to:

   RRA_PROG_LD_FLAG([-fPIE], [-pie],
                    [CFLAGS="${CFLAGS} -fPIE"
                     LDFLAGS="${LDFLAGS} -pie"], [])

instead of only using RRA_PROG_CC_FLAG, checking that the compiler 
supports -fPIE, and assuming (without testing it) that the linker will 
accept -pie.

-- 
Julien ÉLIE

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