ident: add NO_GECOS_IN_PWENT for systems without pw_gecos in struct passwd

Allow NO_GECOS_IN_PWENT to be defined in the Makefile for platforms that
lack the pw_gecos field in their "struct passwd", in which case the
uppercased user name is used instead via the standard '&' replacement
mechanism.

Signed-off-by: Rafael Gieschke <rafael@gieschke.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -59,6 +59,9 @@ all::
 #
 # Define NO_MKSTEMPS if you don't have mkstemps in the C library.
 #
+# Define NO_GECOS_IN_PWENT if you don't have pw_gecos in struct passwd
+# in the C library.
+#
 # Define NO_LIBGEN_H if you don't have libgen.h.
 #
 # Define NEEDS_LIBGEN if your libgen needs -lgen when linking

ident.c

@@ -9,6 +9,12 @@
 
 static char git_default_date[50];
 
+#ifdef NO_GECOS_IN_PWENT
+#define get_gecos(ignored) "&"
+#else
+#define get_gecos(struct_passwd) ((struct_passwd)->pw_gecos)
+#endif
+
 static void copy_gecos(const struct passwd *w, char *name, size_t sz)
 {
 	char *src, *dst;
@@ -20,7 +26,7 @@ static void copy_gecos(const struct passwd *w, char *name, size_t sz)
 	 * with commas.  Also & stands for capitalized form of the login name.
 	 */
 
-	for (len = 0, dst = name, src = w->pw_gecos; len < sz; src++) {
+	for (len = 0, dst = name, src = get_gecos(w); len < sz; src++) {
 		int ch = *src;
 		if (ch != '&') {
 			*dst++ = ch;