build: make outputs independent of the checkout path

[xrviv]

Summary

Make GDK’s C++ and Rust outputs independent of the absolute checkout path.

This fixes #225 and allows third parties to reproduce GDK without recreating Blockstream’s CI directory structure.

Problem

GDK embedded its absolute source directory into compiled output.

Using release_0.77.6, the same source, builder image, dependencies, and build command produced different Android libraries at different checkout paths:

Checkout path	libgreen_gdk_java.so SHA-256
/root/gdk	b60ce800…
/root/gdk/gdk	440ca6ed…
/builds/blockstream/gdk	fbcd5f11…
/builds/blockstream/green/gdk	13d4ba62…

Only the final path reproduced the published library.

Changes

• Add -ffile-prefix-map=${CMAKE_SOURCE_DIR}=. to supported C and C++ builds.
• Pass the top-level source root into the Rust build script.
• Add Rust’s --remap-path-prefix without discarding inherited flags.
• Preserve Android’s required -L…/libgcc flag.
• Respect CARGO_ENCODED_RUSTFLAGS when Cargo uses it instead of RUSTFLAGS.
• Pass --locked to Cargo, as requested in Android release_0.77.6 tarball does not reproduce from documented GitLab CI recipe #225.

The C++ and Rust changes are separate commits. I kept --locked in a third commit because dependency locking has a separate rationale, but I am happy to fold it into the Rust commit if preferred.

Verification

Tested commit 4cf8443ad17aaae66a13a0de2b5f927b925c3d9c with the pinned builder:

docker.io/blockstream/gdk-android-builder@sha256:e08f5a77cc5472c9539cd0a83ee376efffab02eff4e768eae2c16c243f587f1e

I made two clean arm64 Android Release builds with identical inputs but different checkout paths:

• Build A: /builds/blockstream/green/gdk
• Build B: /root/gdk

All four outputs were byte-for-byte identical:

Output	SHA-256 for both builds
src/libgreen_gdk.a	48ce7e4313decc8220b0479032632a77b7b79e73292619ee147475ecb88d1ab0
gdk-rust/libgdk_rust.a	a2157fcadb08e9990e277e43a64e8c7819450665faeab6e6439d71c9d7471f45
libgreen_gdk_java.so	1aa34e6fd3f565f759350909d16ee57ee58aa4c4d709f5b211b7b49a4515644e
libgreen_gdk_java.syms	48b8dcb14034b1c0b7eb52c86e4f838d5958348923e01271add8201875c99ac5

Additional checks:

• Neither absolute checkout path remained in the four outputs.
• Neither path remained in the tested C/C++ object files.
• Verbose rustc output contained the correct --remap-path-prefix for each build.
• Android’s -L…/libgcc flag remained present.
• Both builds used --locked.
• The .syms file still resolved a machine address to a real function and source line.

The patched hashes are intentionally different from the old release hash because path remapping changes the recorded strings. The relevant result is that the two patched builds match each other.

Scope

The acceptance test covers the arm64 Android Release build. No wallet behavior, protocol handling, or key-management code is changed.