libtextstyle: Reexport isatty() override from gnulib.
This is necessary for programs that are compiled for native Windows and
run in a Cygwin console.
* libtextstyle/autogen.sh (GNULIB_MODULES): Add 'isatty'.
* libtextstyle/configure.ac: Set GNULIB_ISATTY to 0.
* libtextstyle/lib/textstyle.h (libtextstyle_isatty): New declaration.
(isatty): Redirect to libtextstyle_isatty.
