Do not use __builtin_types_compatible_p in C++ mode (bug 21930)

The logic to define isinf for float128 depends on the availability of
__builtin_types_compatible_p, which is only available in C mode,
however, the conditionals do not check for C or C++ mode.  This lead to
an error in libstdc++ configure, as reported by bug 21930.

This patch adds a conditional for C mode in the definition of isinf for
float128.  No definition is provided in C++ mode, since libstdc++
headers undefine isinf.

Tested for powerpc64le (glibc test suite and libstdc++-v3 configure).

	[BZ #21930]
	* math/math.h (isinf): Check if in C or C++ mode before using
	__builtin_types_compatible_p, since this is a C mode feature.

1 files changed, 6 insertions, 2 deletions

diff --git a/math/math.h b/math/math.h
index e21708045a..dea8dbe1ae 100644
--- a/math/math.h
+++ b/math/math.h
@@ -442,8 +442,12 @@ enum
 
 /* Return nonzero value if X is positive or negative infinity.  */
 # if __HAVE_DISTINCT_FLOAT128 && !__GNUC_PREREQ (7,0) \
-     && !defined __SUPPORT_SNAN__
-   /* __builtin_isinf_sign is broken for float128 only before GCC 7.0.  */
+     && !defined __SUPPORT_SNAN__ && !defined __cplusplus
+   /* Since __builtin_isinf_sign is broken for float128 before GCC 7.0,
+      use the helper function, __isinff128, with older compilers.  This is
+      only provided for C mode, because in C++ mode, GCC has no support
+      for __builtin_types_compatible_p (and when in C++ mode, this macro is
+      not used anyway, because libstdc++ headers undefine it).  */
 #  define isinf(x) \
     (__builtin_types_compatible_p (__typeof (x), _Float128) \
      ? __isinff128 (x) : __builtin_isinf_sign (x))