Rcpp and default C++ compiler

Question

I have some strange troubles with Rcpp - it uses unpredictable C++ compiler. This question is somewhat similar to this question.
I'm on OSX, I have 2 complilers - default clang and clang-omp with openmp support. Also I have following ~/.R/Makevars file (where I set up clang-omp as default compiler):

CC=clang-omp
CXX=clang-omp++
CFLAGS += -O3 -Wall -pipe -pedantic -std=gnu99
CXXFLAGS += -O3 -Wall -pipe -Wno-unused -pedantic -fopenmp

The problem is that, the package I'm developing compiles with clang++, not clang-omp++. I also tried (as experiment to lacate issue) to change package src/Makevars and set CXX=clang-omp++ and moreover modified $R_HOME/etc/Makeconf CXX entry to CXX = clang-omp++. No luck - it still compiles with clang++. Have no idea why it happens.

Also here is small reproducible (both from console R and from Rstudio) example (don't know whether it related to issue above). Suppose 2 very similar cpp functions:
1.

#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericVector timesTwo(NumericVector x) {
  return x * 2;
}  

Call sourceCpp from R:

library(Rcpp)  
sourceCpp("src/Rcpp_compiler.cpp", verbose = T)

/Library/Frameworks/R.framework/Resources/bin/R CMD SHLIB -o 'sourceCpp_1.so' 'Rcpp_compiler.cpp'
clang-omp++ -I/Library/Frameworks/R.framework/Resources/include -DNDEBUG -I/usr/local/include -I/usr/local/include/freetype2 -I/opt/X11/include -I"/Users/dmitryselivanov/Library/R/3.2/library/Rcpp/include" -I"/Users/dmitryselivanov/projects/experiments/src" -fPIC -Wall -mtune=core2 -g -O2 -O3 -Wall -pipe -Wno-unused -pedantic -fopenmp -c Rcpp_compiler.cpp -o Rcpp_compiler.o
clang-omp++ -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/Library/Frameworks/R.framework/Resources/lib -L/usr/local/lib -o sourceCpp_1.so Rcpp_compiler.o -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation

Work as expected - uses clang-omp++ and all my flags from ~/.R/Makevars

2.

#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::plugins(cpp11)]]
// [[Rcpp::export]]
NumericVector timesTwo(NumericVector x) {
  return x * 2;
}  

Library/Frameworks/R.framework/Resources/bin/R CMD SHLIB -o 'sourceCpp_2.so' 'Rcpp_compiler.cpp'
clang++ -std=c++11 -I/Library/Frameworks/R.framework/Resources/include -DNDEBUG -I/usr/local/include -I/usr/local/include/freetype2 -I/opt/X11/include -I"/Users/dmitryselivanov/Library/R/3.2/library/Rcpp/include" -I"/Users/dmitryselivanov/projects/experiments/src" -fPIC -Wall -mtune=core2 -g -O2 -c Rcpp_compiler.cpp -o Rcpp_compiler.o
clang++ -std=c++11 -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/Library/Frameworks/R.framework/Resources/lib -L/usr/local/lib -o sourceCpp_2.so Rcpp_compiler.o -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation

I only added // [[Rcpp::plugins(cpp11)]] and it compiles with clang++ instead of clang-omp++

Here is my sessionInfo():

R version 3.2.1 (2015-06-18) Platform: x86_64-apple-darwin13.4.0 (64-bit)
Running under: OS X 10.10.5 (Yosemite)
locale: 1 en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8 attached base packages:
1 stats graphics grDevices utils datasets methods base
other attached packages: 1 Rcpp_0.12.1
loaded via a namespace (and not attached): 1 tools_3.2.1

Answer 1

Thanks, to @Dirk hint, I finaly got an answer. Hope, this will save a little bit of time for somebody. Following two lines in ~/.R/Makevars solved my problem:

CXX1X=clang-omp++

See details in this Writing R Extensions section.