Resolve conflicts for functions introduced in Android API level 21. * lib/unistd.in.h (dup3): Consider REPLACE_DUP3. Disable _GL_CXXALIASWARN invocation on non-glibc systems. * m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Initialize REPLACE_DUP3. * modules/unistd (Makefile.am): Substitute REPLACE_DUP3. * m4/dup3.m4 (gl_FUNC_DUP3): Conditionally set REPLACE_DUP3.