koga: give installed binaries an rpath pointing at the install prefix

[dg1sbg]

Fixes #1831 — installed binaries keep loading libclasp out of the build tree.

The bug

The link step emits one rpath: the absolute build lib directory. install copies the binary but cannot rewrite an rpath baked in at link time, so an installed clasp resolves libclasp through the build tree.

$ otool -l <prefix>/bin/iclasp | grep -A2 LC_RPATH
    path /path/to/clasp/build/boehmprecise/lib
$ DYLD_PRINT_LIBRARIES=1 <prefix>/bin/clasp --version
dyld[...] /path/to/clasp/build/boehmprecise/lib/libclasp.dylib

Even with a single install this means removing the build tree breaks the install, rebuilding it silently changes which clasp the installed binary runs, and the libclasp copied into the prefix is never loaded. With two installs neither works: both resolve to whatever the build tree currently holds, so each reports the other's version. I found it exactly that way.

The change

Emit a loader-relative rpath ahead of the absolute one:

#+darwin "-Wl,-rpath,@loader_path/../lib"
#-darwin "-Wl,-rpath,'$$ORIGIN/../lib'"

layout	binary	library	../lib resolves to
install	<prefix>/bin/iclasp	<prefix>/lib	<prefix>/lib ✓
build	<build>/<variant>/iclasp	<build>/<variant>/lib	<build>/lib — absent, skipped

So one link serves both: installed binaries prefer their own prefix, and the in-tree binary falls through to the absolute entry exactly as before. No install-time rewriting, and no dependency on install_name_tool or patchelf (the latter is often not installed).

$ORIGIN must survive two layers to reach the linker — ninja ($$) and /bin/sh (single quotes). Confirmed via ninja -t commands:

-Wl,-rpath,'$ORIGIN/../lib'

Verification

macOS (arm64, LLVM 22):

$ otool -l build/boehmprecise/iclasp | grep -A2 LC_RPATH
    path @loader_path/../lib
    path /path/to/clasp/build/boehmprecise/lib
in-tree binary:            ok 42, loads .../build/boehmprecise/lib/libclasp.dylib
copy in install layout:    loads /tmp/rpx/lib/libclasp.dylib

x86-64 Linux (LLVM 18, bytecode):

$ readelf -d build/boehmprecise/iclasp | grep RUNPATH
  Library runpath: [$ORIGIN/../lib:/mnt/.../build/boehmprecise/lib]
in-tree binary:            intree-ok 42
copy in install layout:    /tmp/rpx/bin/../lib/libclasp.so   (build tree still present)

In both cases the install-layout copy prefers its own prefix while the build tree exists, and the in-tree binary is unchanged.

One thing worth your eye

The relative entry is inert in the build tree only because <build>/lib does not exist. That holds for every variant today, but it is a load-bearing assumption: a future layout that creates build/lib would make the in-tree binary prefer it. If you would rather not depend on that, the alternative is rewriting the rpath during install (what CMake does), at the cost of requiring install_name_tool/patchelf — happy to do it that way instead.

The reproducible-build branch of that cond is untouched; it already remaps paths for its own reasons and I did not want to disturb it without understanding the intent.