Fix: force _lttng python binding to be linked with g++

Fix: force _lttng python binding to be linked with g++

Observed issue
==============

On Enterprise Linux 7 CI nodes, several tests using the python binding
were failing with errors such as the following:

```
ERROR: ust/exit-fast/test_exit-fast
===================================

Warning: Failed to produce a random seed using getrandom(), falling back to pseudo-random device seed generation which will block until its pool is initialized: getrandom() is not supported by this platform [getrandom_nonblock() random.cpp:90]
Traceback (most recent call last):
File "/home/jenkins/workspace/lttng-tools_master_elbuild/babeltrace_version/stable-2.0/build/std/conf/std/liburcu_version/master/platform/el7-amd64/src/lttng-tools/extras/bindings/swig/python/lttng.py", line 24, in swig_import_helper
fp, pathname, description = imp.find_module('_lttng', [dirname(__file__)])
File "/usr/lib64/python3.6/imp.py", line 297, in find_module
raise ImportError(_ERR_MSG.format(name), name=name)
ImportError: No module named '_lttng'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "./ust/exit-fast/test_exit-fast.py", line 19, in <module>
from test_utils import *
File "/home/jenkins/workspace/lttng-tools_master_elbuild/babeltrace_version/stable-2.0/build/std/conf/std/liburcu_version/master/platform/el7-amd64/src/lttng-tools/tests/utils/test_utils.py", line 24, in <module>
from lttng import *
File "/home/jenkins/workspace/lttng-tools_master_elbuild/babeltrace_version/stable-2.0/build/std/conf/std/liburcu_version/master/platform/el7-amd64/src/lttng-tools/extras/bindings/swig/python/lttng.py", line 34, in <module>
_lttng = swig_import_helper()
File "/home/jenkins/workspace/lttng-tools_master_elbuild/babeltrace_version/stable-2.0/build/std/conf/std/liburcu_version/master/platform/el7-amd64/src/lttng-tools/extras/bindings/swig/python/lttng.py", line 26, in swig_import_helper
import _lttng
ImportError: /home/jenkins/workspace/lttng-tools_master_elbuild/babeltrace_version/stable-2.0/build/std/conf/std/liburcu_version/master/platform/el7-amd64/src/lttng-tools/extras/bindings/swig/python/.libs/_lttng.so: undefined symbol: _ZNSt13runtime_errorC2EPKc
ERROR: ust/exit-fast/test_exit-fast - missing test plan
```

The link mode can be seen the commands use to do the linking in the CI
node build logs. For example,

```
libtool: link: gcc -shared  -fPIC -DPIC  .libs/lttng_wrap.o  -Wl,--whole-archive ../../../../src/common/.libs/libsessiond-comm.a ../../../../src/common/.libs/libcommon-gpl.a -Wl,--no-whole-archive  -Wl,-rpath -Wl,/home/jenkins/workspace/lttng-tools_master_elbuild/babeltrace_version/stable-2.0/build/std/conf/std/liburcu_version/master/platform/el8-amd64/src/lttng-tools/src/lib/lttng-ctl/.libs -Wl,-rpath -Wl,/build/lib64 -L/home/jenkins/workspace/lttng-tools_master_elbuild/babeltrace_version/stable-2.0/build/std/conf/std/liburcu_version/master/platform/el8-amd64/deps/build/lib64 ../../../../src/lib/lttng-ctl/.libs/liblttng-ctl.so -lxml2 -L/build/lib64 -lurcu -lurcu-common -lurcu-cds -lrt  -pthread -g -O2   -pthread -Wl,-soname -Wl,_lttng.so.0 -o .libs/_lttng.so.0.0.0
```

Cause
=====

Automake chooses the link mode based on the types of files in the
library or executable. Given that the generated bindings are only C
code, automake uses the gcc link mode.

Solution
========

By adding a dummy (non existant) C++ source file to the library,
automake can be 'forced' to the switch the link mode to `g++`.

Example link command in `g++` mode:

```
libtool: link: g++-10 -std=gnu++11  -fPIC -DPIC -shared -nostdlib
/usr/lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/10/crtbeginS.o  .libs/lttng_wrap.o
-Wl,--whole-archive ../../../../src/common/.libs/libsessiond-comm.a
../../../../src/common/.libs/libcommon-gpl.a -Wl,--no-whole-archive
-Wl,-rpath
-Wl,/home/kstewart/src/efficios/lttng/master/src/lttng-tools/src/lib/lttng-ctl/.libs
-Wl,-rpath -Wl,/home/kstewart/src/efficios/lttng/master/usr/lib
-Wl,-rpath -Wl,/home/kstewart/src/efficios/lttng/master/usr/lib
../../../../src/lib/lttng-ctl/.libs/liblttng-ctl.so -lxml2
-L/home/kstewart/src/efficios/lttng/master/usr/lib
/home/kstewart/src/efficios/lttng/master/usr/lib/liburcu.so
/home/kstewart/src/efficios/lttng/master/usr/lib/liburcu-cds.so
/home/kstewart/src/efficios/lttng/master/usr/lib/liburcu-common.so
-lrt -L/usr/lib/gcc/x86_64-linux-gnu/10
-L/usr/lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/10/../../../../lib
-L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/10/../../.. -lstdc++
-lm -lc -lgcc_s /usr/lib/gcc/x86_64-linux-gnu/10/crtendS.o
/usr/lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crtn.o  -g
-O2 -fuse-ld=lld   -pthread -Wl,-soname -Wl,_lttng.so.0 -o
.libs/_lttng.so.0.0.0
```

Known drawbacks
===============

None.

Change-Id: I5f1dedec435089518e36cc12cd09c2bb151adb67
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>