Don’t use alloc_size with xlclang 16.1
* m4/gnulib-common.m4 (_GL_HAS_ATTRIBUTE): Require Clang 5 instead
of 3.5 for this, to pacify xlclang 16.1.0 on AIX 7.2, which
- advertises itself as clang 4.1.0, and which otherwise issues
+ advertises itself as clang 4.0.1, and which otherwise issues
warnings like “./xalloc.h:141:3: warning: 1540-2990 The attribute
"__attribute__((alloc_size(2, 3)))" is not supported. The
attribute is ignored.” when building bleeding-edge GNU grep.