[2.26 COMMITTED] Do not use generic selection in C++ mode

[Florian Weimer]

The logic to protect the use of generic selection (_Generic) does not
check for C or C++ mode, however, generic selection is a C-only
feature.

Tested for powerpc64le.

	* misc/sys/cdefs.h (__HAVE_GENERIC_SELECTION): Define to 0, if
	in C++ mode.

(cherry picked from commit 6913ad65e00bb32417ad39c41d292b976171e27e)

2017-08-18  Gabriel F. T. Gomes  <gftg@linux.vnet.ibm.com>

	* misc/sys/cdefs.h (__HAVE_GENERIC_SELECTION): Define to 0, if
	in C++ mode.

diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index 06523bfe9c..0c808216a4 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -464,17 +464,18 @@
 # define __glibc_macro_warning(msg)
 #endif
 
-/* Support for generic selection (ISO C11) is available in GCC since
-   version 4.9.  Previous versions do not provide generic selection,
-   even though they might set __STDC_VERSION__ to 201112L, when in
-   -std=c11 mode.  Thus, we must check for !defined __GNUC__ when
-   testing __STDC_VERSION__ for generic selection support.
+/* Generic selection (ISO C11) is a C-only feature, available in GCC
+   since version 4.9.  Previous versions do not provide generic
+   selection, even though they might set __STDC_VERSION__ to 201112L,
+   when in -std=c11 mode.  Thus, we must check for !defined __GNUC__
+   when testing __STDC_VERSION__ for generic selection support.
    On the other hand, Clang also defines __GNUC__, so a clang-specific
    check is required to enable the use of generic selection.  */
-#if __GNUC_PREREQ (4, 9) \
-    || __glibc_clang_has_extension (c_generic_selections) \
-    || (!defined __GNUC__ && defined __STDC_VERSION__ \
-	&& __STDC_VERSION__ >= 201112L)
+#if !defined __cplusplus \
+    && (__GNUC_PREREQ (4, 9) \
+	|| __glibc_clang_has_extension (c_generic_selections) \
+	|| (!defined __GNUC__ && defined __STDC_VERSION__ \
+	    && __STDC_VERSION__ >= 201112L))
 # define __HAVE_GENERIC_SELECTION 1
 #else
 # define __HAVE_GENERIC_SELECTION 0