…aethersdr#3165)

## Summary

Distros that participate in
[reproducible-builds.org](https://reproducible-builds.org/) (Debian,
Arch, NixOS, openSUSE, Fedora, etc.) set `SOURCE_DATE_EPOCH` before
invoking cmake to make `__DATE__` / `__TIME__` deterministic. GCC 7.2+
and Clang [already honour the env
var](https://gcc.gnu.org/onlinedocs/cpp/Environment-Variables.html) at
the compiler level — no source-tree changes are needed for AetherSDR to
produce reproducible builds under these packagers' standard pipelines.

This change is just a configure-time status line so packagers can
confirm at a glance that their env-var actually reached the build:

\`\`\`
$ SOURCE_DATE_EPOCH=\$(git log -1 --format=%ct) cmake -B build -S .
…
-- Build SHA: 8b0c2d5
-- Reproducible build: honoring SOURCE_DATE_EPOCH=1717238400
…
\`\`\`

Unset → no diff vs. main.

## Context

Closes the ecosystem-standard alternative to aethersdr#3139, where @dawkagaming
proposed a project-specific `BUILD_TIMESTAMPS` opt-out flag to suppress
\`__DATE__\` for reproducibility. The counter-proposal in aethersdr#3139's review
thread laid out why `SOURCE_DATE_EPOCH` is preferable:

1. **Universal**: every packager already knows it from non-AetherSDR
work
2. **No source-file \`#ifdef\` branches** — the toolchain handles
substitution
3. **Preserves real compile-date info** for end users; no \`"---"\`
placeholders in About
4. **Covers more than \`__DATE__\`** — \`__TIME__\`, \`ar\`/\`ld\` link
timestamps, deterministic-archive tooling all honour it
5. **Doesn't conflate the git SHA** — the SHA is already deterministic
from the source tree

@dawkagaming closed aethersdr#3139 in favour of this approach. Credit to him for
surfacing the reproducibility problem in the first place — the
implementation took a different shape but the diagnosis was correct.

## Test plan

- [x] Configure without \`SOURCE_DATE_EPOCH\`: no diff vs. main (no
status line, no behavior change)
- [x] Configure with \`SOURCE_DATE_EPOCH=1717238400\`: prints
"Reproducible build: honoring …"
- [x] Full build (\`--target AetherSDR\`) clean
- [x] No source-tree changes (no \`#ifdef\`s, no preprocessor branching)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>