Always include the up-to-date abbreviated git hash if we are in the git repository.

Makefile.am

@@ -12,8 +12,10 @@ shairport_sync_SOURCES = build_details.h shairport.c rtsp.c mdns.c common.c rtp.
 
 
 # from Stack Overflow, with thanks: https://stackoverflow.com/questions/54303270/how-to-execute-shell-commands-in-automake-makefile-am
-build_details.h:
-	echo "#define GITDESCRIPTION \"`git describe --dirty --always 2>/dev/null`\"" >$@
+build_details.h: FORCE
+	( [ -e $(CURDIR)/.git ] && echo "#define GITDESCRIPTION \"`git describe --dirty --always 2>/dev/null`\"" >$@ ) || echo "" >$@
+FORCE:
+
 if BUILD_FOR_FREEBSD
   AM_CXXFLAGS = -I/usr/local/include -Wno-multichar -Wall -Wextra -pthread -DSYSCONFDIR=\"$(sysconfdir)\"
   AM_CFLAGS = -Wno-multichar -Wall -Wextra -pthread -DSYSCONFDIR=\"$(sysconfdir)\"

common.c

@@ -1284,10 +1284,12 @@ char *get_version_string() {
   char *version_string = malloc(1024);
   if (version_string) {
     strcpy(version_string, PACKAGE_VERSION);
+#ifdef GITDESCRIPTION
     if (strlen(GITDESCRIPTION)) {
       strcat(version_string, "-");
       strcat(version_string, GITDESCRIPTION);
     }
+#endif
 #ifdef CONFIG_LIBDAEMON
   strcat(version_string, "-libdaemon");
 #endif