Add compatibility layer for C++11 mode

Add compatibility layer for C++11 mode

Introduce a compatibility layer for C++11 code by adding
include/abg-cxx-compat.h. abg-cxx-compat defines a new namespace
abg_compat and defines
	abg_compat::hash
	abg_compat::shared_ptr
	abg_compat::weak_ptr
	abg_compat::dynamic_pointer_cast
	abg_compat::static_pointer_cast
	abg_compat::unordered_map
	abg_compat::unordered_set
based on definitions from std::tr1 (std=gnu++98) or std:: (std=gnu++11).

I decided for introducing abg_compat:: rather than polluting abigail::
to allow an easier transition to C++11 at a later time and to not subtly
break existing code.

As the shared_ptr in C++11 defines shared_ptr::operator bool() explicit,
some locations where a shared_ptr is assigned to boolean, needed to be
adjusted to explicitly cast to bool.

	* include/abg-cxx-compat.h: new file introducing the abg_compat
	  namespace to provide C++11 functionality from either std::tr1
	  or std::
	* include/Makefile.am: Add the new abg-cxx-compat.h to source
	  distribution.
	* include/abg-comparison.h: replace std::tr1 usage by abg_compat
	  and adjust includes accordingly: likewise
	* include/abg-diff-utils.h: likewise
	* include/abg-fwd.h: likewise
	* include/abg-ini.h: likewise
	* include/abg-interned-str.h: likewise
	* include/abg-ir.h: likewise
	* include/abg-libxml-utils.h: likewise
	* include/abg-libzip-utils.h: likewise
	* include/abg-reporter.h: likewise
	* include/abg-sptr-utils.h: likewise
	* include/abg-suppression.h: likewise
	* include/abg-tools-utils.h: likewise
	* include/abg-workers.h: likewise
	* src/abg-comp-filter.cc: likewise
	* src/abg-comparison-priv.h: likewise
	* src/abg-corpus.cc: likewise
	* src/abg-dwarf-reader.cc: likewise
	* src/abg-hash.cc: likewise
	* src/abg-ir.cc: likewise
	* src/abg-reader.cc: likewise
	* src/abg-suppression.cc: likewise
	* src/abg-tools-utils.cc: likewise
	* src/abg-writer.cc: likewise
	* tests/test-diff-filter.cc: likewise
	* tests/test-diff-pkg.cc: likewise
	* tests/test-read-dwarf.cc: likewise
	* tests/test-read-write.cc: likewise
	* tests/test-types-stability.cc: likewise
	* tests/test-write-read-archive.cc: likewise
	* tools/abicompat.cc: likewise
	* tools/abidiff.cc: likewise
	* tools/abidw.cc: likewise
	* tools/abilint.cc: likewise
	* tools/abipkgdiff.cc: likewise

Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>