Conditionally define R_NO_REMAP and STRICT_R_HEADERS (#410)

* Conditionally define `R_NO_REMAP` and `STRICT_R_HEADERS`

* NEWS bullet

* Fill in number

Author: DavisVaughan

NEWS.md

@@ -1,5 +1,9 @@
 # cpp11 (development version)
 
+* `R_NO_REMAP` and `STRICT_R_HEADERS` are now conditionally defined only if they
+  have not already been defined elsewhere. This is motivated by the fact that
+  `R_NO_REMAP` is becoming the default for C++ code in R 4.5.0 (#410).
+
 * Because cpp11 now requires R >=4.0.0 and `R_UnwindProtect()` is always
   available, `HAS_UNWIND_PROTECT` is no longer useful. Please avoid using it,
   as we'd like to remove it in the future (#411).

inst/include/cpp11/R.hpp

@@ -8,8 +8,14 @@
 #endif
 #endif
 
+#ifndef R_NO_REMAP
 #define R_NO_REMAP
+#endif
+
+#ifndef STRICT_R_HEADERS
 #define STRICT_R_HEADERS
+#endif
+
 #include "R_ext/Boolean.h"
 #include "Rinternals.h"
 #include "Rversion.h"