doc: Use DYLD_LIBRARY_PATH on macOSX

doc: Use DYLD_LIBRARY_PATH on macOSX

Sphinx fails on python bindings documentation generation:

  PYTHONPATH="/Users/jenkins/test/babeltrace/src/bindings/python/bt2/build/build_lib" LD_LIBRARY_PATH="/Users/jenkins/test/babeltrace/src/lib/.libs" /opt/local/bin/python3 -m sphinx -b html -E ./source build/html
  Running Sphinx v2.1.2

  Configuration error:
  There is a programmable error in your configuration file:

  Traceback (most recent call last):
    File "/Users/jenkins/test/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/native_bt.py", line 14, in swig_import_helper
      return importlib.import_module(mname)
    File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/importlib/__init__.py", line 127, in import_module
      return _bootstrap._gcd_import(name[level:], package, level)
    File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
    File "<frozen importlib._bootstrap>", line 983, in _find_and_load
    File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
    File "<frozen importlib._bootstrap>", line 670, in _load_unlocked
    File "<frozen importlib._bootstrap>", line 583, in module_from_spec
    File "<frozen importlib._bootstrap_external>", line 1043, in create_module
    File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  ImportError: dlopen(/Users/jenkins/test/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/_native_bt.cpython-37m-darwin.so, 2): Library not loaded: /usr/local/lib/libbabeltrace2.0.dylib
    Referenced from: /Users/jenkins/test/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/_native_bt.cpython-37m-darwin.so
    Reason: image not found

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
    File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/sphinx/config.py", line 361, in eval_config_file
      execfile_(filename, namespace)
    File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 86, in execfile_
      exec(code, _globals)
    File "/Users/jenkins/test/babeltrace/doc/bindings/python/source/conf.py", line 3, in <module>
      import bt2
    File "/Users/jenkins/test/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/__init__.py", line 26, in <module>
      from bt2.clock_class import *
    File "/Users/jenkins/test/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/clock_class.py", line 23, in <module>
      from bt2 import native_bt, object, utils
    File "/Users/jenkins/test/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/native_bt.py", line 17, in <module>
      _native_bt = swig_import_helper()
    File "/Users/jenkins/test/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/native_bt.py", line 16, in swig_import_helper
      return importlib.import_module('_native_bt')
    File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/importlib/__init__.py", line 127, in import_module
      return _bootstrap._gcd_import(name[level:], package, level)
  ModuleNotFoundError: No module named '_native_bt'

Sphinx simply fails to find libbabeltrace2.0.dylib since that
on macOSX LD_LIBRARY_PATH have no meaning for the dynamic linker,
DYLD_LIBRARY_PATH does.

We could also use DYLD_FALLBACK_LIBRARY_PATH.

The dyld manual have this note regarding System Integrity Protection
(SIP).

  Note: If System Integrity Protection is enabled, these environment
  variables are ignored when executing binaries protected by System
  Integrity Protection.

This patch was tested on our builder and with SIP enabled without any
problem:
  ci-node-macosx-01:babeltrace jenkins$ system_profiler SPSoftwareDataType
    Software:

    System Software Overview:

      System Version: macOS 10.14.5 (18F203)
      Kernel Version: Darwin 18.6.0
      Boot Volume: Macintosh HD
      Boot Mode: Normal
      Computer Name: ci-node-macosx-01
      User Name: Jenkins CI (jenkins)
      Secure Virtual Memory: Enabled
      System Integrity Protection: Enabled
      Time since boot: 32 days 3:36

This mostly works because the python3 executed is not flagged
restricted:
  ci-node-macosx-01:~ jenkins$ ls -laO /opt/local/bin/python3
  lrwxr-xr-x  1 root  admin  - 24 27 Jun 14:16 /opt/local/bin/python3 -> /opt/local/bin/python3.7

The same command would most probably not work if using a
"system" python.
  ci-node-macosx-01:~ jenkins$ ls -laO /usr/bin/python
  -rwxr-xr-x  1 root  wheel  restricted,compressed 66880  4 May 02:49 /usr/bin/python

I was unable to test this scenario.

User building on macOSX could face this issue in the future. I am not sure
how to mitigate this.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: Ia828ac133e2c05b614427f03064c8db02f34f741
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1652
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>