fix for pr100017

gistfile1.txt

diff --git a/libstdc++-v3/include/c_compatibility/fenv.h b/libstdc++-v3/include/c_compatibility/fenv.h
index 0413e3b7c25..6b491ecf003 100644
--- a/libstdc++-v3/include/c_compatibility/fenv.h
+++ b/libstdc++-v3/include/c_compatibility/fenv.h
@@ -26,6 +26,10 @@
  *  This is a Standard C++ Library header.
  */
 
+#if !defined __cplusplus || defined _GLIBCXX_INCLUDE_NEXT_C_HEADERS
+# include_next <fenv.h>
+#else
+
 #ifndef _GLIBCXX_FENV_H
 #define _GLIBCXX_FENV_H 1
 
@@ -79,3 +83,5 @@ namespace std
 #endif // C++11
 
 #endif // _GLIBCXX_FENV_H
+
+#endif // !defined __cplusplus || defined _GLIBCXX_INCLUDE_NEXT_C_HEADERS
diff --git a/libstdc++-v3/include/c_global/cfenv b/libstdc++-v3/include/c_global/cfenv
index 0b0ec35a837..d24cb1a3c81 100644
--- a/libstdc++-v3/include/c_global/cfenv
+++ b/libstdc++-v3/include/c_global/cfenv
@@ -37,9 +37,11 @@
 
 #include <bits/c++config.h>
 
-#if _GLIBCXX_HAVE_FENV_H
-# include <fenv.h>
-#endif
+// Need to ensure this finds the C library's <fenv.h> not a libstdc++
+// wrapper that might already be installed later in the include search path.
+#define _GLIBCXX_INCLUDE_NEXT_C_HEADERS
+#include_next <fenv.h>
+#undef _GLIBCXX_INCLUDE_NEXT_C_HEADERS
 
 #ifdef _GLIBCXX_USE_C99_FENV_TR1