do commit hash replacement in Makefile.am and improve fallback

Signed-off-by: Anton Semjonov <anton@semjonov.de>

Author: ansemjo

.gitattributes

@@ -1 +1 @@
-main.c export-subst
+Makefile.am export-subst

Makefile.am

@@ -66,9 +66,15 @@ libqemu_slirp_a_SOURCES = \
 
 libslirp_a_SOURCES = rd235_libslirp/src/qemu2libslirp.c
 
-VERSION := $(shell [ -d .git ] && echo "-DVERSION=\\\"$$(git describe --always --abbrev)\\\"")
-
-slirp4netns_CFLAGS = $(AM_CFLAGS) $(VERSION)
+# define specific version if commit information available or was
+# replaced during git-archive creation
+VERSION := $(shell V=$Format:%h$ ; \
+	expr match "$$V" ormat: >/dev/null \
+		&& ([ -d .git ] && git describe --always --abbrev || echo unknown) \
+		|| echo "$$V" )
+DEFINE_VERSION = -DVERSION="\"$(VERSION)\""
+
+slirp4netns_CFLAGS = $(AM_CFLAGS) $(DEFINE_VERSION)
 slirp4netns_SOURCES = main.c slirp4netns.c
 slirp4netns_LDADD = libslirp.a libqemu_slirp.a
 man1_MANS = slirp4netns.1

main.c

@@ -233,8 +233,9 @@ static int parent(int sock, int exit_fd, unsigned int mtu, bool enable_ipv6)
 	return 0;
 }
 
+// fallback if VERSION is undefined
 #ifndef VERSION
-#define VERSION "$Format:%h$"
+#define VERSION "unknown"
 #endif
 
 static void usage(const char *argv0)
@@ -246,8 +247,7 @@ static void usage(const char *argv0)
 	printf("-r, --ready-fd=FD    specify the FD to write to when the network is configured\n");
 	printf("-m, --mtu=MTU        specify MTU (default=1500, max=65521)\n");
 	printf("-6, --enable-ipv6    enable IPv6 (experimental)\n");
-	printf("\n");
-	printf("Version: %s\n", VERSION);
+	printf("\nVersion: %s\n", VERSION);
 }
 
 struct options {