koga: make ninja install produce a usable libclasp.pc

[dg1sbg]

Two independent defects in koga's pkg-config handling, one commit each. Together they mean ninja install cannot complete on macOS, and that the pkg-config file it installs cannot link anything on any platform.

1. pkgconfig-path defaults into a SIP-protected directory

bin-path, lib-path, dylib-path and share-path all default under /usr/local, but pkgconfig-path defaults to /usr/lib/pkgconfig/. On macOS Catalina and later /usr/lib is on the sealed read-only system volume and is protected by System Integrity Protection, so ninja -C build install ends with:

[1/1] Installing libclasp.pc to /usr/lib/pkgconfig/libclasp.pc
FAILED: [code=71] /usr/lib/pkgconfig/libclasp.pc
install -C -m 644 libclasp.pc /usr/lib/pkgconfig/libclasp.pc
install: /usr/lib/pkgconfig/INS@na30mW: Operation not permitted
ninja: build stopped: subcommand failed.

sudo does not help — the EPERM is SIP, not ownership; root has no access to that directory either. So ninja install cannot run to completion out of the box on any recent Mac.

Changed the default to /usr/local/lib/pkgconfig/, which is in pkg-config's default pc_path on both Linux and macOS and is consistent with the other four install paths.

2. The installed libclasp.pc cannot link anything

print-prologue (:libclasp-pc) passed "" where write-pc expects the Libs prefix, producing Libs: -lclasp with no library search path:

$ clang++ t.cc -o t $(pkg-config --libs libclasp)
ld: library 'clasp' not found

The per-variant .pc (:libclasp-pc-variant) does pass the variant ldflags, so only the installed copy was affected.

-L alone is not sufficient on macOS. otool -D shows libclasp.dylib's install name is @rpath/libclasp.dylib, so a consumer that now links successfully still dies at startup:

dyld: Library not loaded: @rpath/libclasp.dylib
  Reason: no LC_RPATH's found

so the fix emits -Wl,-rpath, as well, and the flags work end to end.

dylib-path had no entry in *root-paths* — :install-bin, :install-share and :install-lib are all present — so this adds :install-dylib alongside them. It deliberately uses the unresolved install path rather than the --package-path-resolved one, matching :install-share directly above: a staged package must still describe where the files will finally live.

One point you may want to push back on: putting -Wl,-rpath, in Libs: is the pragmatic fix. The alternative is to give the installed dylib an absolute -install_name so no rpath is needed at all, but that is a larger change and would diverge from the build tree, which relies on @rpath. Happy to split the rpath half out or rework it that way if you prefer.

Testing

macOS 15 / Apple Silicon, :build-mode :native, install prefix /opt/clasp.

Before: ninja install fails on the .pc; with the file installed by hand, pkg-config --libs libclasp yields -lclasp and linking fails with ld: library 'clasp' not found.

After:

$ pkg-config --libs libclasp
-L/opt/clasp/lib/ -Wl,-rpath,/opt/clasp/lib/ -lclasp -L.../gmp/lib -lgmpxx -lgmp -L.../fmt/lib -lfmt

$ clang++ t.cc -o t $(pkg-config --libs libclasp) && ./t && echo ok
ok

$ otool -l t | grep -A2 LC_RPATH
          cmd LC_RPATH
      cmdsize 32
         path /opt/clasp/lib/ (offset 12)

ninja -C build install then completes with exit 0, and a second run reports no work to do.

The new default was verified independently by removing the local config.sexp override and re-running ./koga: build.ninja then targets /usr/local/lib/pkgconfig/libclasp.pc.

Neither change triggers a rebuild. Regenerating with ./koga touches only build.ninja — koga's timestamp-preserving output streams leave config.h and the generated headers byte-identical, so ninja -n install afterwards shows a single pending edge.