RcppParallel 6.1.0

• RcppParallel now provides isProcessForkedChild() (R) and
  RcppParallel::isProcessForkedChild() (C++), which return TRUE when the
  current process is a fork() of the process in which RcppParallel was
  loaded.
  Packages dispatching parallel work from within parallel::mclapply() (or
  similar) should consult this and fall back to a serial path, as TBB does
  not support use after fork. (#243, #244)

• The TBB backend is now enabled by default on musl-based Linux
  distributions, such as Alpine Linux. Previously, TBB was only enabled by
  default with glibc-based toolchains on Linux. (#231, #263)

• The mingw cpuid guard applied to TBB's _machine.h header during
  installation is now logged, and a warning is emitted if the header does
  not have the expected form and the guard cannot be applied. (#265)

• On Linux, the bundled TBB libraries are once again installed with versioned
  names (e.g. libtbb.so.2) plus an unversioned libtbb.so symlink, matching
  the layout shipped by RcppParallel 5.1.11 and earlier. The oneTBB cmake build
  produces only unversioned libraries on Linux, so binaries compiled against
  those releases (which recorded a load-time dependency on libtbb.so.2) would
  otherwise fail to load after an upgrade with "libtbb.so.2: cannot open shared
  object file". (#260)

• Fixed linking of downstream packages using the TBB scalable allocator
  on Windows, e.g. via RcppArmadillo's ARMA_USE_TBB_ALLOC. RcppParallel
  now links the whole Rtools tbbmalloc archive into RcppParallel.dll
  and re-exports its API, so that scalable_malloc, scalable_free, and
  friends can be resolved by packages linking with -lRcppParallel. (#262)

• Fixed installation on Windows toolchains providing an older (non-oneTBB)
  copy of TBB, e.g. Rtools42: the tbb stub library is now built by
  re-exporting the static TBB library, rather than wrapping the oneTBB
  runtime (which is unavailable there). In addition, stale stub build
  artifacts from a different toolchain are no longer reused. (#258)

• Fixed installation on Windows systems whose Rtools does not provide TBB
  (R < 4.2.0): configure no longer requires cmake there, and the tbb stub
  library is no longer built when the TBB backend is disabled. (#257)

• RcppParallel now reports which TBB headers and libraries are installed
  with the package, and from where, during package installation. In
  addition, setting the VERBOSE environment variable to a value other
  than 0 enables diagnostics describing how TBB libraries are resolved
  and loaded when the package is loaded. (#256, #259)

• On Windows, RcppParallel once again loads its compatibility stub library
  (tbb.dll) when the package is loaded. Packages linking with -ltbb
  (e.g. via StanHeaders) record a load-time dependency on tbb.dll, which
  can only be resolved if RcppParallel has already loaded it; with
  RcppParallel 6.0.0, such packages would fail to load with "LoadLibrary
  failure: The specified module could not be found". (#249, #250)

• The TBB headers installed with RcppParallel (whether from Rtools or from
  the bundled copy of oneTBB) now guard against GCC's <cpuid.h> being
  included before <intrin.h> on Windows (mingw). Previously, translation
  units including <cpuid.h> before any TBB header would fail to compile,
  as the __cpuid macro from <cpuid.h> conflicts with the __cpuid()
  function declared by mingw's <intrin.h>. (#248, #253)

• When building the bundled copy of oneTBB, RcppParallel no longer searches
  for hwloc, and so no longer tries to build the optional 'tbbbind' library.
  This fixes build failures on machines where a static hwloc library is
  discoverable via pkg-config, as on the CRAN macOS machines. (#247)

• On macOS, the bundled copy of oneTBB is now built with
  __TBB_RESUMABLE_TASKS_USE_THREADS, avoiding use of the deprecated
  ucontext APIs (getcontext, swapcontext, makecontext). (#247)