stdioext: Avoid compilation errors on UnixWare 7. Reported by Tim Rice <tim@multitalents.net> in <https://lists.gnu.org/archive/html/bug-gnulib/2020-09/msg00127.html>. * lib/fbufmode.c: Don't include <stdio_ext.h> if it does not exist. * lib/fpurge.c: Likewise. * lib/freadable.h: Likewise. * lib/freading.h: Likewise. * lib/fwritable.h: Likewise. * lib/fwriting.h: Likewise. * m4/fbufmode.m4 (gl_FUNC_FBUFMODE): Test whether <stdio_ext.h> exists. * m4/fpurge.m4 (gl_FUNC_FPURGE): Likewise. * m4/freadable.m4 (gl_FUNC_FREADABLE): Likewise. * m4/freading.m4 (gl_FUNC_FREADING): Likewise. * m4/fwritable.m4 (gl_FUNC_FWRITABLE): Likewise. * m4/fwriting.m4 (gl_FUNC_FWRITING): Likewise.