Fix warnings for functions introduced in Android API level 16 or 17. * m4/posix_memalign.m4 (gl_FUNC_POSIX_MEMALIGN): Test for posix_memalign using gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE. * m4/alignalloc.m4 (gl_ALIGNALLOC): Likewise. * m4/pagealign_alloc.m4 (gl_PREREQ_PAGEALIGN_ALLOC): Likewise. * modules/aligned-malloc (configure.ac): Likewise.