[libc] avoid c++ includes from MPFR

Author: SchrodingerZhu

libc/utils/MPFRWrapper/mpfr_inc.h

@@ -17,7 +17,21 @@
 // MPFR header can be included in manner allowed in that repo.
 #include "CustomMPFRIncluder.h"
 #else
+
+extern "C" {
+#pragma push_macro("__cplusplus")
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wbuiltin-macro-redefined"
+#endif
+#undef __cplusplus
 #include <mpfr.h>
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+#pragma pop_macro("__cplusplus")
+}
+
 #endif
 
 #endif // LLVM_LIBC_UTILS_MPFRWRAPPER_MPFR_INC_H