[R Package] Use the C++ 11 compiler to test OpenMP flags

[yixuan]

According to the discussion in #1812 , the wrong OpenMP was passed to GCC on Solaris. The reason is that when running the configure script, the default compiler (Solaris Studio) was used, while GCC is the C++ 11 compiler to build the package. Therefore we need to set the CXX environment variable to the C++ 11 compiler in configure.

[codecov-io]

Current coverage is 33.89% (diff: 100%)

Merging #1881 into master will increase coverage by 0.03%

@@             master      #1881   diff @@
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  Files            68         68          
  Lines          5281       5281          
  Methods         827        827          
  Messages          0          0          
  Branches        526        526          
==========================================
+ Hits           1788       1790     +2   
  Misses         3415       3415          
+ Partials         78         76     -2   

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[thirdwing]

When I submit the script early today, I also have these lines.

I removed them because on my solaris VM, it doesn't change the generated Makevars.

I think I know what's wrong now. The R from CSW (https://www.opencsw.org/packages/CSWr-base/) is configured in a different way from CRAN machine.

R version 3.3.0 (2016-05-03) -- "Supposedly Educational"
Copyright (C) 2016 The R Foundation for Statistical Computing
Platform: i386-pc-solaris2.10 (32-bit)

It seems I need to compile R from source on Solaris.

@hetong007 Please merge this and submit again.