Fix llvm-ar detection for macOS builds

The original assumed that Clang always provides llvm-ar, which fails on
macOS where Apple Clang is the default compiler but doesn't include
llvm-ar. This causes build failures when ENABLE_EXT_F=1.

Add runtime detection using 'which llvm-ar' to check availability before
using it. Falls back to system 'ar' when llvm-ar is not found, ensuring
builds work correctly with both Apple Clang and Homebrew LLVM.

Author: jserv

Makefile

@@ -116,7 +116,11 @@ $(call set-feature, EXT_F)
 ifeq ($(call has, EXT_F), 1)
 AR := ar
 ifeq ("$(CC_IS_CLANG)", "1")
-AR = llvm-ar
+    # Check if llvm-ar is available
+    LLVM_AR := $(shell which llvm-ar 2>/dev/null)
+    ifneq ($(LLVM_AR),)
+        AR = llvm-ar
+    endif
 endif
 ifeq ("$(CC_IS_EMCC)", "1")
 AR = emar