Fix version header generation for dkms

The dkms build fails for the latest release:

  #60

The problem is that dkms does not call my Makefile at all, so the
version header is never generated. Dkms does have a PRE_BUILD hook
though, so use that to call a script that takes care of this.

From now on, I must always remember to run a dkms build before a new
release.

Signed-off-by: Ernesto A. Fernández <ernesto@corellium.com>

Makefile

@@ -6,7 +6,6 @@
 KERNELRELEASE ?= $(shell uname -r)
 KERNEL_DIR    ?= /lib/modules/$(KERNELRELEASE)/build
 PWD           := $(shell pwd)
-GIT_COMMIT    = $(shell git describe HEAD | tail -c 9)
 
 obj-m = apfs.o
 apfs-y := btree.o compress.o dir.o extents.o file.o inode.o key.o libzbitmap.o \
@@ -15,7 +14,7 @@ apfs-y := btree.o compress.o dir.o extents.o file.o inode.o key.o libzbitmap.o \
 	  spaceman.o super.o symlink.o transaction.o unicode.o xattr.o xfield.o
 
 default:
-	@printf '#define GIT_COMMIT\t"%s"\n' $(GIT_COMMIT) > version.h
+	./genver.sh
 	make -C $(KERNEL_DIR) M=$(PWD)
 install:
 	make -C $(KERNEL_DIR) M=$(PWD) modules_install

dkms.conf

@@ -5,3 +5,4 @@ BUILT_MODULE_NAME[0]="apfs"
 DEST_MODULE_LOCATION[0]="/extra"
 
 AUTOINSTALL="yes"
+PRE_BUILD="genver.sh"

genver.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Script to generate the module version header
+#
+
+if command -v git >/dev/null 2>&1; then
+	GIT_COMMIT=$(git describe HEAD | tail -c 9)
+else
+	GIT_COMMIT="unknown"
+fi
+
+printf '#define GIT_COMMIT\t"%s"\n' "$GIT_COMMIT" > version.h