undo #define copysign in pyconfig.h

include/pybind11/detail/common.h

diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index bb1affc..e2330bb 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -113,6 +113,9 @@
 #include <frameobject.h>
 #include <pythread.h>
 
+/* Python #defines overrides on all sorts of core functions, which
+   tends to weak havok in C++ codebases that expect these to work
+   like regular functions (potentially with several overloads) */
 #if defined(isalnum)
 #  undef isalnum
 #  undef isalpha
@@ -123,6 +126,10 @@
 #  undef toupper
 #endif
 
+#if defined(copysign)
+#  undef copysign
+#endif
+
 #if defined(_MSC_VER)
 #  if defined(PYBIND11_DEBUG_MARKER)
 #    define _DEBUG