ship versioned tbb libraries on linux again

[kevinushey]

Problem

On Linux, RcppParallel 6.0.x installs unversioned TBB libraries — libtbb.so with SONAME libtbb.so, and no libtbb.so.2. This is a regression from 5.1.11 and earlier, which shipped a real libtbb.so.2 plus a libtbb.so redirect.

The cause is the oneTBB cmake build: it only versions its output under if (WIN32) (OUTPUT_NAME "tbb${TBB_BINARY_VERSION}") and sets no SOVERSION, so Linux/macOS get the bare target name. 5.1.11's versioned layout was never something RcppParallel did explicitly — it came from Intel TBB's make-based build (build/linux.inc set the SONAME suffix from TBB_COMPATIBLE_INTERFACE_VERSION, i.e. 2, and Makefile.tbb emitted libtbb.so as an INPUT(libtbb.so.2) linker script).

Binaries compiled against RcppParallel ≤ 5.1.11 recorded a load-time dependency on libtbb.so.2. After an upgrade to 6.0.x that file no longer exists, so they fail to load:

unable to load shared object '.../foo.so':
  libtbb.so.2: cannot open shared object file: No such file or directory

This was reported by the CRAN maintainer during revdep checking on Fedora.

Fix

For the bundled TBB on Linux only, after copying the libraries, rename each real unversioned libtbb*.so to libtbb*.so.2 and re-create libtbb*.so as a relative symlink pointing back at it. This restores the historical layout so legacy NEEDED libtbb.so.2 dependencies resolve again.

• Scoped to the bundled build; the external/system-TBB path is untouched (that library keeps its own SONAME and lives on a stable system path).
• Idempotent: skips symlinks/linker scripts and any already-versioned file.
• A symlink, not a linker script: a linker script (INPUT(...)) works at link time but the dynamic loader can't parse it, so it would fail at runtime — and 6.0.x's RcppParallel.so itself has NEEDED libtbb.so.

Verification

Rebuilt into a temporary library on Linux:

• Resulting layout is libtbb.so.2 (real) + libtbb.so -> libtbb.so.2 for tbb, tbbmalloc, and tbbmalloc_proxy.
• Package loads; setThreadOptions() works; tbbLibraryPath("tbb") returns the .so.2.
• RcppParallel.so's own NEEDED libtbb.so resolves via the new symlink.
• A synthesized binary with a genuine NEEDED libtbb.so.2 (mimicking a 5.1.11-built revdep) now resolves the file against the new install — the load error is gone.

Note

Giving the new-ABI oneTBB the legacy .so.2 name means an old binary now loads it and could fault on a removed symbol rather than getting a clean file-not-found; that is what the existing compatibility headers/symbols are for, and a rebuild remains the real fix. This change restores pre-6.0 behavior and stops the file-level breakage.