Ensure a consistent C++ standard use

Ensure a consistent C++ standard use

On older compilers (such as g++ 4.8), the default C++ standard is set to
gnu++98. When compiling libabigail with --enable-cxx11=yes, src/ and
tests/ where compiled with the correct flag, while tools/ was compiled
without specifying a standard. With a compiler falling back to gnu++98
that leads to unresolved references when linking the tools against the
libabigail library. Fix that by consistently using the std= flag across
the code base.

	* configure.ac: add -std=c++11 flag to CXXFLAGS when compiling
	for C++11
	* src/Makefile.am: drop now obsolete setting of the -std flag
	* tests/Makefile.am: likewise

Reported-by: Chun-Hung Wu <Chun-hung.Wu@mediatek.com>
Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>