remove references to c++11 in Makevars

[mcshaz]

Adressing issue #5

This seems to address compiler warnings and fits my intepretation of the suggestion to simply not suggest a c++ standard for the compiler.

Still a few warnings to potentially check:

racusum_mc.cpp:364:21: warning: comparison of integer expressions of different signedness: 'int' and 'const arma::uword' {aka 'const long long unsigned int'} [-Wsign-compare]

and

racusum_mc.cpp: In function 'double racusum_crit_mc(Rcpp::NumericMatrix, double, double, double, double, int, int, int, bool)':
racusum_mc.cpp:543:20: warning: 'h' may be used uninitialized [-Wmaybe-uninitialized]
  543 |   if ( L1 < L0 ) h = h + 1 / pow(static_cast<double>(10), jmax);