Taken from  https://gcc.gnu.org/cgi-bin/get-raw-msg?listname=gcc-patches&date=2016-01&msgid=1451802493-17406-1-git-send-email-vapier%40gentoo.org

Since the 3.0.3 release of gperf (made in May 2007), the generated func
has had the gnu_inline attribute applied to it.  The gcc source however
has not been updated to include that which has lead to a mismatch.

In practice, this hasn't been an issue for two reasons:
(1) Before gcc-5, the default standard was (gnu) C89, and gcc does not
warn or throw an error in this mode.
(2) Starting with gcc-4.8, the compiler driver used to build gcc was
changed to C++, and g++ does not warn or throw an error in this mode.

This error does show up though when using gcc-5 to build gcc-4.7 or
older as then the default is (gnu) C11 and the C compiler driver is
used.  That failure looks like:
In file included from .../gcc-4.7.4/gcc/cp/except.c:990:0:
cfns.gperf: At top level:
cfns.gperf:101:1: error: 'gn</td></tr></table>
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<div class='content'><div id='summary'>d00c0e 100644
--- a/gcc/cp/cfns.h
+++ b/gcc/cp/cfns.h
@@ -53,6 +53,9 @@ __inline
 static unsigned int hash (const char *, unsigned int);
 #ifdef __GNUC__
 __inline
+#ifdef __GNUC_STDC_INLINE__
+__attribute__ ((__gnu_inline__))
+#endif
 #endif
 const char * libc_name_p (const char *, unsigned int);
 /* maximum key range = 391, duplicates = 0 */
-- 
2.6.2