libutils: Add WEAK_ALIAS attribute macro

Incorrect test of __clang__ in preprocessor directive

Align checks of the __clang__ flag so that it is tested in the same way
in all preprocessor directives. Setting __clang__ to 0 would result in
an unexpected result.

fix licenses

libutils: Document min and max constructor priorities

libutils: Add constructor attribute

libutils: trivial: Replace ifdef guards with pragma once.

libutils: Abstract 'hot' attribute.

libutils: Abstract 'cold' attribute.

libutils: fix: remove duplicated __has_attribute stub.

libutils: Provide a __has_feature stub for GCC.

As per previous commits, this is related to compatibility with Clang.

libutils: Support for Clang in the ERROR macro.

`ERROR` is a wrapper around GCC's error attribute, but this feature does not
exist on Clang. Clang has an unavailable attribute with similar semantics, so
we map it to this.

libutils: Provide a __has_extension stub for GCC.

This macro is provided by Clang to test for support of specific language
extensions. We can't use it on GCC, even when guarded by an "#ifdef __clang__"
because GCC tries to evaluate it. To get around this we define it to always
fail on GCC.

Add 'section' attribute abstraction.