Describe the bug
The shared library build options depend on the system's uname. This especially causes builds to fail when cross compiling from macOS because of the check here:

To Reproduce
Steps to reproduce the behavior:

• Plattform: macOS
• Install arm-none-eabi-gcc: brew install --cask gcc-arm-embedded
• Use the following to build

#!/bin/bash

set -e

export ZSTD_LIB_COMPRESSION=0
export ZSTD_LIB_DECOMPRESSION=1
export ZSTD_LIB_DICTBUILDER=0
export ZSTD_LIB_DEPRECATED=0
export ZSTD_LEGACY_SUPPORT=0

export CC=$(which arm-none-eabi-gcc)
export CFLAGS="-march=armv6k+fp -mtune=mpcore -mfloat-abi=hard -mtp=soft -O3 -fomit-frame-pointer"

make lib-nomt

Expected behavior
Assignment of the shared lib flags should not depend on uname and should be configurable.

Desktop:

• OS: macOS 15.0.1 (24A348)
• Compiler: arm-none-eabi-gcc 14.2.1 20241119 (Arm GNU Toolchain 14.2.Rel1 (Build arm-14.52))
• Flags: see above
• Build system: Makefile