ci : update + packages (#146)

* Add simple Debian package

* Update CI configuration

* Add cpack Deb package for libggwave-dev

* Add `make deb` top level target and CI job

* Ensure `make deb` packages are deterministic

Update CMakeLists.txt to make cpack generated Debian packages
deterministic build outputs by:
- Setting SOURCE_DATE_EPOCH to 0
- Executing `git log -1 --pretty=%ct` and storing the result in
  SOURCE_DATE_EPOCH, overriding 0
- Export SOURCE_DATE_EPOCH to the environment
- Explicitly prefix the execution of the cpack command in the custom
  target `deb_c_library` with SOURCE_DATE_EPOCH

Update `README.md-tmpl.md` and `README.md` to document new `make deb`
target.

The Makefile for the Python bindings already handle deterministic
building with the same strategy.

* libc6, not libc

README.md

@@ -159,6 +159,24 @@ make
 ./bin/ggwave-cli
 ```
 
+#### Local Debian packages
+
+Build reproducible `libggwave-dev` and `python3-ggwave` Debian packages:
+```bash
+# Fetch source
+git clone https://github.com/ggerganov/ggwave --recursive
+cd ggwave
+
+# Configure
+cmake . -DGGWAVE_BUILD_EXAMPLES=OFF -DCMAKE_BUILD_TYPE=Release
+
+# Build
+make deb
+
+# Install
+sudo dpkg -i dist/*.deb
+```
+
 ### Emscripten
 
 ```bash