Fix Makefile errors that prevent builds on macOS (#409)

Makefile

@@ -1,8 +1,7 @@
-DOCKER := "$(shell { command -v podman || command -v docker; })"
-TIMESTAMP := "$(shell date -u +"%Y%m%d%H%M")"
-COMMIT := "$(shell git rev-parse --short HEAD 2>/dev/null)"
-detected_OS := "$(shell uname)"  # Classify UNIX OS
-ifeq ($(strip $(detected_OS)),Darwin) #We only care if it's OS X
+DOCKER := $(shell { command -v podman || command -v docker; })
+TIMESTAMP := $(shell date -u +"%Y%m%d%H%M")
+COMMIT := $(shell git rev-parse --short HEAD 2>/dev/null)
+ifeq ($(shell uname),Darwin)
 SELINUX1 :=
 SELINUX2 :=
 else