man: support bringing your own go-md2man

Set the GO_MD2MAN make variable to elide building go-md2man from
vendored sources and use the specified command instead.

Signed-off-by: Cory Snider <csnider@mirantis.com>
(cherry picked from commit edfde78)
Signed-off-by: Cory Snider <csnider@mirantis.com>

man/Makefile

@@ -3,6 +3,12 @@ mandir = $(prefix)/man
 INSTALL = install
 INSTALL_DATA = ${INSTALL} -m 644
 
+# By default, the man pages are generated using a copy of go-md2man built from
+# the vendored sources in this directory. This behavior can be overridden by
+# setting the GO_MD2MAN variable to the name/path of an existing go-md2man
+# binary.
+GO_MD2MAN ?= .build/go-md2man
+
 ALL_PAGES := $(wildcard *.*.md)
 
 # Determine which manual sections we are generating pages for
@@ -14,8 +20,8 @@ sections := $(sort $(foreach page,$(ALL_PAGES:.md=),$(call man_section,$(page)))
 # Dynamically generate pattern rules for each manual section
 # so make knows how to build a target like man8/dockerd.8.
 define MANPAGE_template
-man$(1)/%.$(1): %.$(1).md .build/go-md2man | man$(1)
-	.build/go-md2man -in $$< -out $$@
+man$(1)/%.$(1): %.$(1).md $(if $(findstring file,$(origin GO_MD2MAN)),$(GO_MD2MAN)) | man$(1)
+	$(GO_MD2MAN) -in $$< -out $$@
 endef
 $(foreach sec,$(sections),$(eval $(call MANPAGE_template,$(sec))))