Setting up Travis CI on Linux with an R package that uses RcppArmadillo

Ralf, thank you! Your advice is making sense.

However, I find that installation on macOS fails when I have this in src/Makevars:

clang++ -std=gnu++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 RcppExports.o harmony.o -L/Library/Frameworks/R.framework/Resources/lib -lRlapack -L/Library/Frameworks/R.framework/Resources/lib -lRblas -L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin15/6.1.0 -L/usr/local/gfortran/lib -lgfortran -lquadmath -lm -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation
ld: warning: directory not found for option '-L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin15/6.1.0'
ld: warning: directory not found for option '-L/usr/local/gfortran/lib'
ld: library not found for -lgfortran
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [] Error 1
ERROR: compilation failed for package ‘harmony’

I find that macOS has no problem with installation when the src/Makevars is empty.

So, it seems that I may need to have one Makevars for macOS, and a different Makevars for Linux.

I'm still scratching my head about how to get this right.

Could I please ask if you know the right way to do this?