Fix warnings for functions introduced in Android API level 26.
* m4/mountlist.m4 (gl_MOUNTLIST): Test for hasmntopt using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS.
* m4/nl_langinfo.m4 (gl_FUNC_NL_LANGINFO): Test for nl_langinfo using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/strtod.m4 (gl_PREREQ_STRTOD): Test for nl_langinfo using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS.
* m4/strtold.m4 (gl_PREREQ_STRTOLD): Likewise.
* m4/vasnprintf.m4 (gl_PREREQ_VASNPRINTF_DIRECTIVE_A): Likewise.
* m4/strerror_r.m4 (gl_PREREQ_STRERROR_R): Test for catgets using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/utimens.m4 (gl_UTIMENS): Test for lutimes using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/duplocale.m4 (gl_FUNC_DUPLOCALE): Test for nl_langinfo_l using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* modules/duplocale-tests (configure.ac): Likewise.
* m4/getdomainname.m4 (gl_FUNC_GETDOMAINNAME): Update comment.