Merge pull request #1356 from buildkite/fix_osx_sed_in_makefile

ensure the SED var is used by inline commands

Author: scadu

Makefile

@@ -74,7 +74,7 @@ build/aws-stack.yml:
 		} else { \
 			print \
 		}\
-	}' templates/aws-stack.yml | sed "s/%v/$(VERSION)/" > $@
+	}' templates/aws-stack.yml | $(SED) "s/%v/$(VERSION)/" > $@
 
 # -----------------------------------------
 # AMI creation with Packer
@@ -116,8 +116,8 @@ build/linux-arm64-ami.txt: packer-linux-arm64.output env-AWS_REGION
 	grep -Eo "$(AWS_REGION): (ami-.+)" $< | cut -d' ' -f2 | xargs echo -n > $@
 
 # NOTE: make removes the $ escapes, everything else is passed to the shell
-CURRENT_AGENT_VERSION_LINUX ?= $(shell sed -En 's/^AGENT_VERSION="?(.+?)"?$$/\1/p' packer/linux/scripts/install-buildkite-agent.sh)
-CURRENT_AGENT_VERSION_WINDOWS ?= $(shell sed -En 's/^\$$AGENT_VERSION = "(.+?)"$$/\1/p' packer/windows/scripts/install-buildkite-agent.ps1)
+CURRENT_AGENT_VERSION_LINUX ?= $(shell $(SED) -En 's/^AGENT_VERSION="?(.+?)"?$$/\1/p' packer/linux/scripts/install-buildkite-agent.sh)
+CURRENT_AGENT_VERSION_WINDOWS ?= $(shell $(SED) -En 's/^\$$AGENT_VERSION = "(.+?)"$$/\1/p' packer/windows/scripts/install-buildkite-agent.ps1)
 
 print-agent-versions:
 	@echo Linux: $(CURRENT_AGENT_VERSION_LINUX)
@@ -235,9 +235,12 @@ AGENT_VERSION ?= $(shell curl -Lfs "https://buildkite.com/agent/releases/latest?
 
 SED ?= sed
 ifeq ($(shell uname), Darwin)
-	# Use GNU sed, not MacOS sed
-	# Install with: brew install gsed
+	# Use GNU sed, not MacOS sed - required for extended regex support
+	# BSD sed (default on macOS) doesn't support the regex patterns used in this Makefile
 	SED = gsed
+	ifeq ($(shell command -v gsed >/dev/null 2>&1 && echo yes || echo no), no)
+    $(error GNU sed (gsed) is required on macOS but not found. Please install with: brew install gnu-sed)
+	endif
 endif
 
 bump-agent-version: