Add C++11 support to the build system

Add C++11 support in preparation to the addition of C++ sources in the
plugins.

Automake uses the following variables for C++ files:

  * AM_CPPFLAGS / CPPFLAGS: Preprocessor flags
  * AM_CXXFLAGS / CXXFLAGS: C++ compiler flags
  * AM_LDFLAGS  / LDFLAGS:  Linker flags

Change-Id: Ib9d1ce4b7853981c441e1c31460fd24579fa97ea
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4413
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>