Version: Ensure "--version" and "version" display a commit id.

git-archive(1), as used by github.com, allows files to be modified
"on-the-fly" as the archive file is generated.

By configuring git-archive to write the commit id to file
"commit_id.fmt", the build can be convinced (using the mandatory
"clean-local" rule) to generate file "commit_id" from "commit_id.fmt".
The value of "commit_id" is then passed via the compiler to the runtime
to allow the latter to display a commit id when run with either
"--version" or "version".

Signed-off-by: James Hunt <[email protected]>

.gitattributes

@@ -0,0 +1,2 @@
+commit_id.fmt   export-subst
+.gitattributes  export-ignore

Makefile.am

@@ -25,14 +25,22 @@ AUTOMAKE_OPTIONS = parallel-tests
 
 CHECK_DEPS =
 
+# Has a value if building in a git tree
 GIT_COMMIT := $(shell git rev-parse HEAD 2>/dev/null)
 
+# Has a value if building in a git-archive(1)-generated tree
+# (see .gitattributes).
+GENERATED_COMMIT = $(shell cat commit_id.fmt 2>/dev/null \| grep \'^[a-f0-9][a-f0-9]*\$\')
+
 commit_id:
 	$(AM_V_GEN)if [ "x$(GIT_COMMIT)" != "x" ]; then \
-		echo "Writing commit id"; \
-		echo $(GIT_COMMIT) > commit_id; \
+		echo "Determining commit id from git"; \
+		echo "$(GIT_COMMIT)" > commit_id; \
+	elif [ "x$(GENERATED_COMMIT)" != "x" ]; then \
+		echo "Determining commit id from generated value"; \
+		echo "$(GENERATED_COMMIT)" > commit_id; \
 	else \
-		echo "WARNING: cannot commit id"; \
+		echo "WARNING: cannot determine actual commit id"; \
 		echo "0" > commit_id; \
 	fi
 

commit_id.fmt

@@ -0,0 +1 @@
+$Format:%H$