libtextstyle: Don't export symbols from static MSVC .obj files.
* libtextstyle/lib/Makefile.am (config.h): Don't alias isatty to
libtextstyle_isatty here, before the Windows header files have been included.
* libtextstyle/gnulib-local/lib/isatty.c.diff: New file.
* libtextstyle/Makefile.am (EXTRA_DIST): Add it.
