Support Intel oneAPI TBB / oneTBB

[hsbadr]

This builds on #141 to support for the latest versions of Intel oneTBB / OneAPI and closes #145.

The latest version of oneAPI/oneTBB redirects the TBB headers to a relative path (#include "../oneapi/tbb.h") and we copy tbb headers in inst/include/tbb without the headers in oneapi. This causes a Catastrophic Error (missing headers) because the relative path doesn't exist.

So, make sure that we copy TBB headers in $TBB_INC/oneapi.

• Updated Makevars.in to copy oneapi TBB headers and fix building errors.
• Added instructions for using external TBB library.
• Updated NEWS.

Tests

To build the development version of RcppParallel with oneTBB or OneAPI:

• Install oneTBB or OneAPI.

For example, installing oneTBB on Linux 64-bit (x86_64) to $HOME directory (change if needed!):

    TBB_VERSION="2021.1.1"

    wget https://github.com/oneapi-src/oneTBB/releases/download/v2021.1.1/oneapi-tbb-$TBB_VERSION-lin.tgz
    tar zxvf oneapi-tbb-$TBB_VERSION-lin.tgz -C $HOME

    export TBB="$HOME/oneapi-tbb-$TBB_VERSION"

• Set the TBB environment variables (specifically: TBB for the installation prefix, TBB_INC for the directory that includes the header files, and TBB_LIB for the libraries directory).

For example, installing oneTBB on Linux 64-bit (x86_64) to $HOME directory (change if needed!):

    source $TBB/env/vars.sh intel64

    export TBB_INC="$TBB/include"
    export TBB_LIB="$TBB/lib/intel64/gcc4.8"
    #export LD_LIBRARY_PATH="$TBB_LIB:$LD_LIBRARY_PATH"

    # Checks:
    ls -lAh $TBB_INC
    # should list `oneapi` and `tbb` directories that include the headers
    ls -lAh $TBB_LIB
    # should list the TBB libraries, including `libtbb.so`

• Build the development version of RcppParallel:

install.packages("remotes")
remotes::install_github("hsbadr/RcppParallel@develop", force = TRUE)

[kevinushey]

LGTM! Thank you for the very thorough pull request, and I especially appreciate the NEWS update.