[PATCH] libstdc++: Don't fail if math_errhandling is not defined

[Matthias Kretz <m.kretz@gsi.de>]

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Tested on x86_64-linux and powerpc64le-linux. This fixes the powerpc64le-linux 
failure Jonathan reported. OK for trunk?


Older glibc does not define math_errhandling with -ffast-math, in which
case floating-point exceptions are not used.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd.h (__floating_point_flags): Do
	not rely on the presence of the math_errhandling macro.
---
 libstdc++-v3/include/experimental/bits/simd.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)


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──────────────────────────────────────────────────────────────────────────
 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 stdₓ::simd
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diff --git a/libstdc++-v3/include/experimental/bits/simd.h b/libstdc++-v3/include/experimental/bits/simd.h
index d837d0efb21..c991e3f223e 100644
--- a/libstdc++-v3/include/experimental/bits/simd.h
+++ b/libstdc++-v3/include/experimental/bits/simd.h
@@ -283,11 +283,20 @@ constexpr inline bool __have_power_vmx = __have_power_vsx;
 
 namespace __detail
 {
+  constexpr bool __handle_fpexcept =
+#ifdef math_errhandling
+    math_errhandling & MATH_ERREXCEPT;
+#elif defined __FAST_MATH__
+    false;
+#else
+    true;
+#endif
+
   constexpr std::uint_least64_t
   __floating_point_flags()
   {
     std::uint_least64_t __flags = 0;
-    if constexpr (math_errhandling & MATH_ERREXCEPT)
+    if constexpr (__handle_fpexcept)
       __flags |= 1;
 #ifdef __FAST_MATH__
     __flags |= 1 << 1;