Add support for a minimal version of libgfortran for accelerator targets.

Add support for a minimal version of libgfortran for accelerator targets.

	* Makefile.am (AM_CFLAGS): Add -DLIBGFOR_MINIMAL if LIBGFOR_MINIMAL.
        (gfor_io_src, gfor_heper_src, gfor_src): Split into minimal and
        always included sources.
        * Makefile.in: Regenerate.
        * configure.ac (LIBGFOR_MINIMAL): New AM_CONDITIONAL.
        * configure: Regenerate.
        * caf/single.c (caf_runtime_error): Don't print messages if
        LIBGFOR_MINIMAL.
        * runtime/compile_options.c (fatal_error_in_progress,
        show_signal, backtrace_handler, maybe_find_addr2line): Guard with
        !defined LIBGFOR_MINIMAL.
        (set_options): Likewise for the backtrace code.
        * runtime/minimal.c: New file.

From-SVN: r218170