Resolve conflicts for functions introduced in Android API level 23. * lib/unistd.in.h (sethostname): Consider REPLACE_SETHOSTNAME. Disable _GL_CXXALIASWARN invocation on non-glibc systems. * m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Initialize REPLACE_SETHOSTNAME. * modules/unistd (Makefile.am): Substitute REPLACE_SETHOSTNAME. * m4/sethostname.m4 (gl_FUNC_SETHOSTNAME): Conditionally set REPLACE_SETHOSTNAME. * modules/sethostname (Depends-on, configure.ac): Consider REPLACE_SETHOSTNAME.