Add support for QNX 7.1 build

[chachoi]

Added QNX build files under the build_qnx folder.

Instructions for building with QNX 7.1:

1. Setup the boost repo:

git clone https://github.com/chachoi/boost.git && cd boost
git checkout develop

2. Replace "url = ../" in .gitmodules with "url = https://github.com/boostorg/". This step won't be necessary once this PR is merged.
3. Init submodules:

git submodule update --init --recursive

4. Change "QCC" in ./tools/build/src/tools/qcc.jam line 41 to "qcc". I submitted a PR for this change. This step won't be necessary once that PR is merged into https://github.com/boostorg/build.

5. Source QNX 7.1 environment script:

source <path-to-script>/qnxsdp-env.sh

6. Build and install

cd build_qnx
make -j 4 install

7. Optional: build a test:

# Example: make test.<lib name> -j 4
make test.math -j 4