Resolve conflicts for functions introduced in Android API level 21. * lib/search.in.h (twalk): Consider REPLACE_TWALK. * m4/search_h.m4 (gl_SEARCH_H_DEFAULTS): Initialize REPLACE_TWALK. * modules/search (Makefile.am): Substitute REPLACE_TWALK. * m4/tsearch.m4 (gl_FUNC_TSEARCH): Conditionally set REPLACE_TWALK. * modules/tsearch (configure.ac): Consider REPLACE_TWALK.