Resolve conflicts for functions introduced in Android API level 21.
* m4/fts.m4 (gl_FUNC_FTS_CORE): Use rpl_* symbols also when the system
will have fts_open in a future version.
* m4/iswblank.m4 (gl_FUNC_ISWBLANK): Conditionally set REPLACE_ISWBLANK.
* m4/mbtowc.m4 (gl_FUNC_MBTOWC): Conditionally set REPLACE_MBTOWC.
* m4/vdprintf.m4 (gl_REPLACE_VDPRINTF): Conditionally set
REPLACE_VDPRINTF.
* m4/wcsnrtombs.m4 (gl_FUNC_WCSNRTOMBS): Conditionally set
REPLACE_WCSNRTOMBS.