build system: Make it possible to build with 64bit time_t

On most 32bit architectures time_t (and a few other time related types)
are a signed 32bit wide integer type.
As a consequence they can only represent dates between

	Fri Dec 13 08:45:52 PM UTC 1901

(-0x80000000 seconds before Jan 1 1970) and

	Tue Jan 19 03:14:07 AM UTC 2038

(0x7fffffff seconds after Jan 1 1970). Given that some machines that are
built today have an expected lifetime of >15 years, this needs to be
extended. To to that, define the cpp symbol _TIME_BITS to 64 which
results in some magic in glibc to make time_t (and the few other time
related types) 64 bit wide.

This new switch CONFIG_TIME64 is in the spirit of CONFIG_LFS and only
expected to have the expected effect with glibc. On musl for examples
time_t already defaults to 64bit wide types.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

Config.in

@@ -108,6 +108,16 @@ config LFS
 	programs that can benefit from large file support include dd, gzip,
 	cp, mount, tar.
 
+config TIME64
+	bool "Support 64bit wide time types"
+	default y
+	depends on LFS
+	help
+	Make times later than 2038 representable for several libc syscalls
+	(stat, clk_gettime etc.). Note this switch is specific to glibc and has
+	no effect on platforms that already use 64bit wide time types (i.e. all
+	64bit archs and some selected 32bit archs (currently riscv and x32)).
+
 config PAM
 	bool "Support PAM (Pluggable Authentication Modules)"
 	default n

Makefile.flags

@@ -15,6 +15,7 @@ CPPFLAGS += \
 	-include include/autoconf.h \
 	-D_GNU_SOURCE -DNDEBUG \
 	$(if $(CONFIG_LFS),-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64) \
+	$(if $(CONFIG_TIME64),-D_TIME_BITS=64) \
 	-DBB_VER=$(squote)$(quote)$(BB_VER)$(quote)$(squote)
 
 CFLAGS += $(call cc-option,-Wall,)