more fun with isinf() and friends.

GNU libstdc++ prior to GCC 6.1.0, the overlay math.h always
includes math.h from glibc, which defines isinf() and friends.

GCC 6.1.0 includes a change (6c8ced3f4f867b72a623fe2f23efa204c5786a28)
so that the overlay math.h never includes the glibc math.h
when compiling c++.
The overlay math.h sometimes includes "using std::isinf"
Determined by inspecting libc math.h when building gcc.

src/libCom/osi/os/posix/epicsMath.h

@@ -18,11 +18,13 @@
 #if __cplusplus>=201103L
 #include <cmath>
 
+#if __GLIBCXX__>20160427
 using std::isfinite;
 using std::isinf;
 using std::isnan;
 using std::isnormal;
 #endif
+#endif /* c++11 */
 
 extern "C" {
 #endif