build: refresh the git description whenever something is recompiled

This fixes https://savannah.gnu.org/bugs/?47962 reported by Cody Taylor.

configure.ac

@@ -708,16 +708,16 @@ AM_CONDITIONAL(GROFF_HTML, test x$groff_html_support = xyes)
 AC_CHECK_PROG(haveit, makeinfo, yes, no)
 AM_CONDITIONAL(HAVE_MAKEINFO, test x$haveit = xyes)
 
+# Check whether this is a git repository.
 AC_MSG_CHECKING([whether building from git])
 if test -d .git ; then
     AC_MSG_RESULT([yes])
-    AC_PATH_PROG([GIT], [git])
-    REVISION=`($GIT rev-parse --is-inside-work-tree >/dev/null 2>&1) && ($GIT rev-parse --short HEAD)`
-    AC_SUBST(REVISION)
-    AC_DEFINE_UNQUOTED([REVISION],"${REVISION}","Setting REVISION in config.h.")
+    from_git=yes
 else
     AC_MSG_RESULT([no])
+    from_git=no
 fi
+AM_CONDITIONAL(BUILDING_FROM_GIT, test x$from_git = xyes)
 
 AC_CONFIG_FILES([
 Makefile

src/Makefile.am

 AM_CPPFLAGS = -DLOCALEDIR=\"$(localedir)\" -DSYSCONFDIR=\"$(sysconfdir)\"
 
+if BUILDING_FROM_GIT
+SOMETHING = "REVISION \"$(shell git describe --tags 2>/dev/null)\""
+else
+SOMETHING = "NOTHING \"from tarball\""
+endif
+
+nano.o: revision.h
+winio.o: revision.h
+
+revision.h: update_revision
+	@[ -f $@ ] || touch $@
+	@echo "#define $(SOMETHING)" | cmp -s $@ - || \
+	echo "#define $(SOMETHING)" > $@
+
+.PHONY: update_revision
+
 bin_PROGRAMS = 	nano
 nano_SOURCES =	browser.c \
 		chars.c \

src/nano.c

@@ -21,6 +21,7 @@
  **************************************************************************/
 
 #include "proto.h"
+#include "revision.h"
 
 #include <stdio.h>
 #include <stdarg.h>
@@ -918,7 +919,7 @@ void usage(void)
 void version(void)
 {
 #ifdef REVISION
-    printf(" nano from git, commit %s (after %s)\n", REVISION, VERSION);
+    printf(" nano from git, %s\n", REVISION);
 #else
     printf(_(" nano, version %s\n"), VERSION);
 #endif

src/nano.h

@@ -604,10 +604,4 @@ enum
 /* The largest size_t number that doesn't have the high bit set. */
 #define HIGHEST_POSITIVE ((~(size_t)0) >> 1)
 
-#ifdef REVISION
-#define BRANDING PACKAGE_VERSION"-git  "REVISION
-#else
-#define BRANDING PACKAGE_STRING
-#endif
-
 #endif /* !NANO_H */

src/winio.c

@@ -21,6 +21,7 @@
  **************************************************************************/
 
 #include "proto.h"
+#include "revision.h"
 
 #include <stdio.h>
 #include <stdarg.h>
@@ -28,6 +29,12 @@
 #include <unistd.h>
 #include <ctype.h>
 
+#ifdef REVISION
+#define BRANDING REVISION
+#else
+#define BRANDING PACKAGE_STRING
+#endif
+
 static int *key_buffer = NULL;
 	/* The keystroke buffer, containing all the keystrokes we
 	 * haven't handled yet at a given point. */