Makefile: Set CGO_CFLAGS dynamically on Mac OS X

When Homebrew is available, gssapi header path can be generated
automatically using `brew --prefix`. This handles all edge cases.

Author: tchap

Makefile

@@ -72,6 +72,14 @@ image-ocp-deployer: image-ocp-cli
 $(call build-image,ocp-recycler,$(IMAGE_REGISTRY)/ocp/4.2:recycler,./images/recycler/Dockerfile.rhel,.)
 image-ocp-recycler: image-ocp-cli
 
+# macOS system GSSAPI headers lack RFC 5587 types (gss_const_name_t).
+# Use Homebrew's heimdal headers when available.
+ifeq ($(shell uname),Darwin)
+ifneq (,$(shell which brew 2>/dev/null))
+build: export CGO_CFLAGS += -I$(shell brew --prefix)/opt/heimdal/include
+endif
+endif
+
 oc: GO_BUILD_PACKAGES :=./cmd/oc
 oc: build
 .PHONY: oc