Rework SOURCE_DATE_EPOCH usage be portable

[bmwiedemann]

Checklist

• Closing issues: #issue
• Mark this if you consider it ready to merge
• I've added tests (optional)
• I wrote some lines in the radare2book

Description

As a follow-up of #19699,
Rework SOURCE_DATE_EPOCH usage in meson to be portable
to make @rofl0r and FreeBSD users happy.

Please test.

[eli-schwartz]

Alternatively, the standard __DATE__ and __TIME__ macros in C could be used GCC since 2017 will respect the SOURCE_DATE_EPOCH environment variable for you and you don't have to do anything in meson.build (in fact you can just remove complexity from both that and configure/make).

[trufae]

What's your proposal then?

[eli-schwartz]

radare2/libr/util/str.c

Line 3917 in ad749a0

s = r_str_appendf (s, "commit: "R2_GITTIP" build: "R2_BIRTH);

radare2/libr/main/radare2.c

Line 675 in 88e8fdf

pj_ks (pj, "birth", R2_BIRTH);

These two locations can be modified to use __DATE__. They will switch from YYYY-MM-DD to MMM DD YYYY, and when SOURCE_DATE_EPOCH is set the compiler (at least GCC) will use that epoch. When it isn't set, or you use some compiler that doesn't itself support the reproducible builds standard (I assume MSVC probably doesn't?) it will just use the time that the compiler was invoked on that file.

[trufae]

I'm fine to merge this PR and i understand your proposal, but im not sure if it will work as its intended to have the same timestamp across all binaries. That would be another behaviour, that maybe we can discuss and use if agreed. Would you like to make a PR to continue the discussion there?