Fix warnings for functions introduced in Android API level 16.
* m4/faccessat.m4 (gl_FUNC_FACCESSAT): Test for faccessat using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/canonicalize.m4 (gl_FUNC_CANONICALIZE_FILENAME_MODE,
gl_CANONICALIZE_LGPL_SEPARATE): Likewise.
* m4/euidaccess.m4 (gl_PREREQ_EUIDACCESS): Likewise.
* m4/tsearch.m4 (gl_FUNC_TSEARCH): Test for tsearch using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS.
* m4/setenv.m4 (gl_PREREQ_SETENV): Likewise.