Move CPP_PROLOGUE and CPP_EPILOGUE to the .cpp

This lets us avoid having to specify them in every C file.

src/jemalloc_cpp.cpp

@@ -2,9 +2,17 @@
 #include <new>
 
 #define JEMALLOC_CPP_CPP_
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include "jemalloc/internal/jemalloc_preamble.h"
 #include "jemalloc/internal/jemalloc_internal_includes.h"
 
+#ifdef __cplusplus
+}
+#endif
+
 // All operators in this file are exported.
 
 // Possibly alias hidden versions of malloc and sdallocx to avoid an extra plt