Rstan C++ compilation error stdlib.h

> system("clang++ -v")
Apple LLVM version 10.0.0 (clang-1000.11.45.2)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
> # check toolchain
> fx <- inline::cxxfunction( signature(x = "integer", y = "numeric" ) , '
+   return ScalarReal( INTEGER(x)[0] * REAL(y)[0] ) ;
+                            ' )
In file included from file823bc75ebc.cpp:3:
In file included from /Library/Frameworks/R.framework/Resources/include/R.h:47:
In file included from /usr/local/clang4/bin/../include/c++/v1/cstdlib:86:
/usr/local/clang4/bin/../include/c++/v1/stdlib.h:94:15: fatal error: 'stdlib.h' file not found
#include_next <stdlib.h>
              ^~~~~~~~~~
1 error generated.
make: *** [file823bc75ebc.o] Error 1

ERROR(s) during compilation: source code errors or compiler configuration errors!

Program source:
  1:
  2: // includes from the plugin
  3: #include <R.h>
  4: #include <Rdefines.h>
  5: #include <R_ext/Error.h>
  6:
  7:
  8: // user includes
  9:
 10:
 11: // declarations
 12: extern "C" {
 13: SEXP file823bc75ebc( SEXP x, SEXP y) ;
 14: }
 15:
 16: // definition
 17:
 18: SEXP file823bc75ebc( SEXP x, SEXP y ){
 19:
 20:    return ScalarReal( INTEGER(x)[0] * REAL(y)[0] ) ;
 21:
 22: Rf_warning("your C++ program does not return anything");
 23:  return R_NilValue ;
 24: }
 25:
 26:
Error in compileCode(f, code, language = language, verbose = verbose) :
  Compilation ERROR, function(s)/method(s) not created! In file included from file823bc75ebc.cpp:3:
In file included from /Library/Frameworks/R.framework/Resources/include/R.h:47:
In file included from /usr/local/clang4/bin/../include/c++/v1/cstdlib:86:
/usr/local/clang4/bin/../include/c++/v1/stdlib.h:94:15: fatal error: 'stdlib.h' file not found
#include_next <stdlib.h>
              ^~~~~~~~~~
1 error generated.
make: *** [file823bc75ebc.o] Error 1
In addition: Warning message:
In system(cmd, intern = !verbose) :
  running command '/Library/Frameworks/R.framework/Resources/bin/R CMD SHLIB file823bc75ebc.cpp 2> file823bc75ebc.cpp.err.txt' had status 1
>