* gthr-gnat.h, gthr-gnat.c: new sources for implementation of
        --enable-threads=gnat.
        * Makefile.in: Add gthr-gnat.c to LIB2ADDEH.
        * configure.in: Add gnat to the list of thread packages
        * configure: Rebuild.
        * config/t-linux: Add gthr-gnat.c to LIB2ADDEH and LIB2ADDEHDEP

From-SVN: r65961