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.
diff --git a/include/haproxy/compiler.h b/include/haproxy/compiler.h
index 6c356a6..82b6ae4 100644
--- a/include/haproxy/compiler.h
+++ b/include/haproxy/compiler.h
@@ -49,6 +49,17 @@
#define ___equals_1(x, ...) ____equals_1(x, 0)
#define __equals_1(x) ___equals_1(comma_for_one ## x 1)
+/* gcc 5 and clang 3 brought __has_attribute(), which is not well documented in
+ * the case of gcc, but is convenient since handled at the preprocessor level.
+ * In both cases it's possible to test for __has_attribute() using ifdef. When
+ * not defined we remap this to the __has_attribute_<name> macro so that we'll
+ * later be able to implement on a per-compiler basis those which are missing,
+ * by defining __has_attribute_<name> to 1.
+ */
+#ifndef __has_attribute
+#define __has_attribute(x) __equals_1(__has_attribute_ ## x)
+#endif
+
#if !defined(__GNUC__)
/* Some versions of glibc irresponsibly redefine __attribute__() to empty for
* non-gcc compilers, and as such, silently break all constructors with other