improve clang support (3)

[Bruno Haible]

clang supports __builtin_expect, even on Windows.

Most Gnulib modules use the 'builtin-expect' module with its Autoconf test,
however, some use the macros from lib/cdefs.h.


2020-08-06  Bruno Haible  <bruno@clisp.org>

        Use __builtin_expect with clang everywhere.
        * lib/cdefs.h (__glibc_unlikely, __glibc_likely): Use the GCC built-in
        also on clang.

diff --git a/lib/cdefs.h b/lib/cdefs.h
index 2158379..b034c0b 100644
--- a/lib/cdefs.h
+++ b/lib/cdefs.h
@@ -401,7 +401,7 @@
 # endif
 #endif
 
-#if __GNUC__ >= 3
+#if (__GNUC__ >= 3) || (__clang_major__ >= 4)
 # define __glibc_unlikely(cond)        __builtin_expect ((cond), 0)
 # define __glibc_likely(cond)  __builtin_expect ((cond), 1)
 #else