Detect when the passwd struct is missing pw_gecos

NO_GECOS_IN_PWENT was documented with other Makefile variables but was only
enforced by manually defining it to the C preprocessor.  This adds support
for detecting the condition with configure and defining the make variable.

Signed-off-by: David Michael <fedora.dm0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1648,6 +1648,9 @@ endif
 ifdef NO_D_INO_IN_DIRENT
 	BASIC_CFLAGS += -DNO_D_INO_IN_DIRENT
 endif
+ifdef NO_GECOS_IN_PWENT
+	BASIC_CFLAGS += -DNO_GECOS_IN_PWENT
+endif
 ifdef NO_ST_BLOCKS_IN_STRUCT_STAT
 	BASIC_CFLAGS += -DNO_ST_BLOCKS_IN_STRUCT_STAT
 endif

configure.ac

@@ -759,6 +759,14 @@ AC_CHECK_MEMBER(struct dirent.d_type,
 [#include <dirent.h>])
 GIT_CONF_SUBST([NO_D_TYPE_IN_DIRENT])
 #
+# Define NO_GECOS_IN_PWENT if you don't have pw_gecos in struct passwd
+# in the C library.
+AC_CHECK_MEMBER(struct passwd.pw_gecos,
+[NO_GECOS_IN_PWENT=],
+[NO_GECOS_IN_PWENT=YesPlease],
+[#include <pwd.h>])
+GIT_CONF_SUBST([NO_GECOS_IN_PWENT])
+#
 # Define NO_SOCKADDR_STORAGE if your platform does not have struct
 # sockaddr_storage.
 AC_CHECK_TYPE(struct sockaddr_storage,