Makefile: Fix native pandoc invocation

In dc9daf9 (Makefile: Replace vbatts/pandoc with a PANDOC variable
2016-05-06, opencontainers#428) I'd misunderstood vbatts/pandoc as a call to a
locally-installed pandoc, when it's actually the name of a Docker
image [1,2].  With this commit, we prefer a local pandoc if one
exists, fall back to Docker and vbatts/pandoc if a local 'docker'
exists, and raise an error if neither 'pandoc' nor 'docker' exist.

[1]: opencontainers#440
[2]: opencontainers#428 (comment)

Reported-by: Qiang Huang <[email protected]>
Reported-by: Lai Jiangshan <[email protected]>
Signed-off-by: W. Trevor King <[email protected]>

Author: wking

Makefile

@@ -1,6 +1,20 @@
 
 DOCKER ?= $(shell command -v docker 2>/dev/null)
 PANDOC ?= $(shell command -v pandoc 2>/dev/null)
+ifeq "$(strip $(PANDOC))" ''
+	ifneq "$(strip $(DOCKER))" ''
+		PANDOC = $(DOCKER) run \
+			-it \
+			--rm \
+			-v $(shell pwd)/:/input/:ro \
+			-v $(shell pwd)/output/:/output/ \
+			-u $(shell id -u) \
+			vbatts/pandoc
+		PANDOC_SRC := /input/
+		PANDOC_DST := /
+	endif
+endif
+
 # These docs are in an order that determines how they show up in the PDF/HTML docs.
 DOC_FILES := \
 	version.md \
@@ -25,25 +39,18 @@ default: docs
 .PHONY: docs
 docs: output/docs.pdf output/docs.html
 
+ifeq "$(strip $(PANDOC))" ''
+output/docs.pdf output/docs.html:
+	$(error cannot build $@ without either pandoc or docker)
+else
 output/docs.pdf: $(DOC_FILES)
 	mkdir -p output/ && \
-	$(DOCKER) run \
-	-it \
-	--rm \
-	-v $(shell pwd)/:/input/:ro \
-	-v $(shell pwd)/output/:/output/ \
-	-u $(shell id -u) \
-	$(PANDOC) -f markdown_github -t latex -o /$@ $(patsubst %,/input/%,$(DOC_FILES))
+	$(PANDOC) -f markdown_github -t latex -o $(PANDOC_DST)$@ $(patsubst %,$(PANDOC_SRC)%,$(DOC_FILES))
 
 output/docs.html: $(DOC_FILES)
 	mkdir -p output/ && \
-	$(DOCKER) run \
-	-it \
-	--rm \
-	-v $(shell pwd)/:/input/:ro \
-	-v $(shell pwd)/output/:/output/ \
-	-u $(shell id -u) \
-	$(PANDOC) -f markdown_github -t html5 -o /$@ $(patsubst %,/input/%,$(DOC_FILES))
+	$(PANDOC) -f markdown_github -t html5 -o $(PANDOC_DST)$@ $(patsubst %,$(PANDOC_SRC)%,$(DOC_FILES))
+endif
 
 code-of-conduct.md:
 	curl -o $@ https://raw.githubusercontent.com/opencontainers/tob/d2f9d68c1332870e40693fe077d311e0742bc73d/code-of-conduct.md