makefile: make STDBUF_CMD optional

originally added for Mac in 4fa7b95, solves problems that don't
exist on x86 Linux distributions normally.

Removing stdbuf dependency is helpful when running in some containerized
environments, like Bazel.

Signed-off-by: Øyvind Harboe <[email protected]>

Author: oharboe

flow/Makefile

@@ -332,7 +332,7 @@ endif
 KLAYOUT_FOUND            = $(if $(KLAYOUT_CMD),,$(error KLayout not found in PATH))
 
 ifneq ($(shell command -v stdbuf),)
-  STDBUF_CMD = stdbuf -o L
+  STDBUF_CMD ?= stdbuf -o L
 endif
 
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