[fix] Added UNAME_TARGET_SYSTEM to make shared lib compile flag selection configurable

[RicBent]

Fixes #4219 by adding UNAME_TARGET_SYSTEM which, when not set, defaults to UNAME.

[Cyan4973]

This is a good stop-gap, it allows a user to manually take control of the situation, without impacting the current default behavior.

I presume that a more robust solution would likely employ the output of the compiler to determine what's the target, but that's more work and more risk, which can be attempted later.

As a side note, UNAME is a variable, so it can already be taken over by the user,
but I guess it's also used in several other places, and you only want to impact this one place.

[RicBent]

Exactly, the UNAME variable is also used to select different tools depending on what OS you are running on, like here:

zstd/lib/libzstd.mk

Line 212 in 0218c8d

ifeq ($(UNAME), Darwin)

[terrelln]

Thanks for the PR @RicBent!