Import selfpipe code from skalibs 1.2.5

[andersk@mit.edu: Move code to new third/libstddjb directory]

third/libstddjb/gccattributes.h

@@ -0,0 +1,61 @@
+/* ISC license. */
+
+#ifndef GCCATTRIBUTES_H
+#define GCCATTRIBUTES_H
+
+#ifdef __GNUC__
+
+#define gccattr_noreturn __attribute__((__noreturn__))
+#define gccattr_noinline __attribute__((__noinline__))
+#define gccattr_inline __attribute__((__always_inline__))
+#define gccattr_const __attribute__((__const__))
+#define gccattr_unused __attribute__((__unused__))
+#define gccattr_used __attribute__((__used__))
+#define gccattr_weak __attribute__((__weak__))
+#define gccattr_aligned __attribute__((__aligned__))
+
+# if (__GNUC__ >= 3) || ((__GNUC__ == 2) && (__GNUC_MINOR__ >= 96))
+#define gccattr_malloc __attribute__((__malloc__))
+#define gccattr_pure __attribute__((__pure__))
+# else
+#define gccattr_malloc
+#define gccattr_pure
+# endif
+
+# if (__GNUC__ >= 3)
+#define gccattr_deprecated __attribute__((__deprecated__))
+# else
+#define gccattr_deprecated
+# endif
+
+#else
+
+#define gccattr_noreturn
+#define gccattr_noinline
+#define gccattr_inline
+#define gccattr_const
+#define gccattr_unused
+#define gccattr_used
+#define gccattr_weak
+#define gccattr_aligned
+#define gccattr_malloc
+#define gccattr_pure
+#define gccattr_deprecated
+
+#endif
+
+#ifdef GCCATTR_COMPAT_0_22
+#define _a_noreturn gccattr_noreturn
+#define _a_noinline gccattr_noinline
+#define _a_inline gccattr_inline
+#define _a_const gccattr_const
+#define _a_unused gccattr_unused
+#define _a_used gccattr_used
+#define _a_weak gccattr_weak
+#define _a_aligned gccattr_aligned
+#define _a_malloc gccattr_malloc
+#define _a_pure gccattr_pure
+#define _a_deprecated gccattr_deprecated
+#endif
+
+#endif