kbuild: mkcompile_h: consider timestamp if KBUILD_BUILD_TIMESTAMP is set

stable inclusion
from stable-5.10.53
commit e378db118925f2fb144e0235334f3e12bd617606
bugzilla: 175574 https://gitee.com/openeuler/kernel/issues/I4DTUX

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e378db118925f2fb144e0235334f3e12bd617606

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[ Upstream commit a979522a ]

To avoid unnecessary recompilations, mkcompile_h does not regenerate
compile.h if just the timestamp changed.
Though, if KBUILD_BUILD_TIMESTAMP is set, an explicit timestamp for the
build was requested, in which case we should not ignore it.

If a user follows the documentation for reproducible builds [1] and
defines KBUILD_BUILD_TIMESTAMP as the git commit timestamp, a clean
build will have the correct timestamp. A subsequent cherry-pick (or
amend) changes the commit timestamp and if an incremental build is done
with a different KBUILD_BUILD_TIMESTAMP now, that new value is not taken
into consideration. But it should for reproducibility.

Hence, whenever KBUILD_BUILD_TIMESTAMP is explicitly set, do not ignore
UTS_VERSION when making a decision about whether the regenerated version
of compile.h should be moved into place.

[1] https://www.kernel.org/doc/html/latest/kbuild/reproducible-builds.html



Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>