Use tar-ustar to avoid problems with a long path in the gtest directory that crosses the 99-char tar-v7 limit if the package version number happens to be long enough.  This was causing make distcheck to fail on release candidates, although it worked fine for final releases.

Go back to prefering -std=gnu++11, but detect the one known case where this breaks and we need to use -std=c++11.

Oops, exporting CXX for AC_CONFIG_SUBDIRS doesn't work if the user specified their own CXX, in which case their version will be in ac_configure_args and will take precedence.  We'll have to modify ac_configure_args.