Drop requirement to compile with GNU extensions

Drop requirement to compile with GNU extensions

__gnu_cxx::stdio_filebuf is a GNU extension only available in certain
std libraries. It is not e.g. in libc++. In order to be able to compile
with using libc++, replace the usage of __gnu_cxx::stdio_filebuf with
standard C++ methods. In this case, reopen the temporary file with a
std::fstream and expose that stream rather than the previously exposed
std::iostream.

	* include/abg-tools-utils.h (get_stream): Change return type to
	  std::fstream
	* src/abg-corpus.cc: remove unused #include of ext/stdio_filebuf.h
	* src/abg-tools-utils (temp_file::priv): remove filebuf_ member,
	  and replace iostream_ by fstream_ with changing the shared_ptr
	  type accordingly
	  (temp_file::priv::priv): initialize fstream_ based on
	  temporary file name
	  (temp_file::priv::~priv): adjust destruction accordingly
	  (temp_file::is_good): test the fstream rather than the fd
	  (temp_file::get_stream): adjust return type to std::fstream
	  and adjust implementation based on the changes in temp_file::priv
	* src/Makefile.am: remove gnu extension from c++ standard flag
	* tests/Makefile.am: likewise

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