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

@@ -42,6 +42,14 @@ GO_BUILD_TAGS_COMMON :=include_gcs include_oss containers_image_openpgp
 GO_BUILD_FLAGS :=-tags '$(GO_BUILD_TAGS_COMMON) gssapi'
 GO_BUILD_FLAGS_CROSS :=-tags '$(GO_BUILD_TAGS_COMMON)'
 
+# 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))
+export CGO_CFLAGS := -I$(shell brew --prefix)/opt/heimdal/include
+endif
+endif
+
 OUTPUT_DIR :=_output
 CROSS_BUILD_BINDIR :=$(OUTPUT_DIR)/bin
 RPM_VERSION :=$(shell set -o pipefail && echo '$(SOURCE_GIT_TAG)' | sed -E 's/v([0-9]+\.[0-9]+\.[0-9]+)-.*/\1/')