avoid collisions with debug namespace

[kevinushey]

Pull Request Template for Rcpp

Closes #1070.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Prefereably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[codecov-io]

Codecov Report

Merging #1071 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1071   +/-   ##
=======================================
  Coverage   95.61%   95.61%           
=======================================
  Files          64       64           
  Lines        2782     2782           
=======================================
  Hits         2660     2660           
  Misses        122      122           

Impacted Files	Coverage Δ
inst/include/Rcpp/exceptions.h	100.00% <ø> (ø)