[libcxx] Do not include the C math.h header before __config

Summary:
Certain C libraries require configuration macros defined in __config
to provide the correct functionality for libc++. This patch ensures
that the C header math.h is always included after the __config
header. It also adds a Windows-specific #if guard for the case when
the C math.h file is included the second time, as suggested by
Marshall in https://reviews.llvm.org/rL323490.

Fixes PR36382.

Reviewers: mclow.lists, EricWF

Reviewed By: mclow.lists

Subscribers: cfe-commits, pcc, christof, rogfer01

Differential Revision: https://reviews.llvm.org/D43579

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@325760 91177308-0d34-0410-b5e6-96231b3b80d8

include/math.h

@@ -8,16 +8,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// This include lives outside the header guard in order to support an MSVC
-// extension which allows users to do:
-//
-// #define _USE_MATH_DEFINES
-// #include <math.h>
-//
-// and receive the definitions of mathematical constants, even if <math.h>
-// has previously been included.
-#include_next <math.h>
-
 #ifndef _LIBCPP_MATH_H
 #define _LIBCPP_MATH_H
 
@@ -308,6 +298,8 @@ long double    truncl(long double x);
 #pragma GCC system_header
 #endif
 
+#include_next <math.h>
+
 #ifdef __cplusplus
 
 // We support including .h headers inside 'extern "C"' contexts, so switch
@@ -1494,4 +1486,18 @@ trunc(_A1 __lcpp_x) _NOEXCEPT {return ::trunc((double)__lcpp_x);}
 
 #endif // __cplusplus
 
+#else // _LIBCPP_MATH_H
+
+// This include lives outside the header guard in order to support an MSVC
+// extension which allows users to do:
+//
+// #define _USE_MATH_DEFINES
+// #include <math.h>
+//
+// and receive the definitions of mathematical constants, even if <math.h>
+// has previously been included.
+#if defined(_LIBCPP_MSVCRT) && defined(_USE_MATH_DEFINES)
+#include_next <math.h>
+#endif
+
 #endif  // _LIBCPP_MATH_H