Fix warnings for functions introduced in Android API level 23.
* m4/fbufmode.m4 (gl_FUNC_FBUFMODE): Test for __flbf, __fbufsize using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/filenamecat.m4 (gl_FILE_NAME_CONCAT_LGPL): Test for mempcpy using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/fpurge.m4 (gl_FUNC_FPURGE): Test for __fpurge using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/freadable.m4 (gl_FUNC_FREADABLE): Test for __freadable using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/fwritable.m4 (gl_FUNC_FWRITABLE): Test for __fwritable using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/getpass.m4 (gl_FUNC_GETPASS_GNU): Test for __fsetlocking using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/mempcpy.m4 (gl_FUNC_MEMPCPY): Test for mempcpy using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS.
* m4/mkfifoat.m4 (gl_FUNC_MKFIFOAT): Test for mkfifoat using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/mkostemp.m4 (gl_FUNC_MKOSTEMP): Test for mkostemp using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/mkostemps.m4 (gl_FUNC_MKOSTEMPS): Test for mkostemps using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/pty.m4 (gl_FUNC_LOGIN_TTY): Test for login_tty using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS.
* m4/sethostname.m4 (gl_FUNC_SETHOSTNAME): Test for sethostname using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS.
* m4/strerror_r.m4 (gl_FUNC_STRERROR_R_WORKS): Test for strerror_r using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/wmempcpy.m4 (gl_FUNC_WMEMPCPY): Test for wmempcpy using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.