BUILD: compiler: add a default definition for __has_attribute()

BUILD: compiler: add a default definition for __has_attribute()

It happens that gcc since 5.x has this macro which is only mentioned
once in the doc, associated with __builtin_has_attribute(). Clang had
it at least since 3.0. In addition it validates #ifdef when present,
so it's easy to detect it. Here we're providing a fallback to another
macro __has_attribute_<name> so that it's possible to define that macro
to the value 1 for older compilers when the attribute is supported.