protect r87449 against Rcpps long-standing undo Rmath`

git-svn-id: https://svn.r-project.org/R/trunk@87453 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: maechler

src/include/Rmath.h0.in

@@ -349,6 +349,7 @@ double  Rlog1p(double);
 #define trigamma	Rf_trigamma
 
 #ifdef  __cplusplus
+#define RCPP_SUGAR_UNDORMATH_H /* temporary only, remove before release ! */
 extern "C" {
 #endif
 	/* R's versions with !R_FINITE checks */