Clang support for gnu_inline attribute

Clang also supports the gnu_inline attribute and the
__GNUC_STDC_INLINE__ macro (C99 semantics). However, it reports as
GCC 4.2 compatible (__GNUC_MINOR__ / __GNUC__) and thus the current
defines do not think it can support this.

Add clang as an alternative for this support. Documentation shows that
this attribute is supported since at least Clang 8.

Signed-off-by: Marcus Haehnel <marcus.haehnel@kernkonzept.com>

include/sys/cdefs.h

@@ -320,7 +320,7 @@
    inline semantics, unless -fgnu89-inline is used.
    For -std=gnu99, forcing gnu_inline attribute does not change behavior,
    but may silence spurious warnings (such as in GCC 4.2).  */
-#if !defined __cplusplus || __GNUC_PREREQ (4,3)
+#if !defined __cplusplus || __GNUC_PREREQ (4,3) || __CLANG_PREREQ(8,0)
 # if defined __GNUC_STDC_INLINE__ || defined __GNUC_GNU_INLINE__ || defined __cplusplus
 #  define __extern_inline extern __inline __attribute__ ((__gnu_inline__))
 #  if __GNUC_PREREQ (4,3)