Kyle, I ran your two test codes on my Windows PC. I got the below results shown on R console.
1, test C++ compiler
c:/Rtools/mingw_64/bin/g++ -I"C:/Users/YSU/R-33~1.1/include" -DNDEBUG -I"d:/Compiler/gcc-4.9.3/local330/include" -O2 -Wall -mtune=core2 -c R_Cpp_Test.cpp -o R_Cpp_Test.o
c:/Rtools/mingw_64/bin/g++ -shared -s -static-libgcc -o R_Cpp_Test.dll tmp.def R_Cpp_Test.o -Ld:/Compiler/gcc-4.9.3/local330/lib/x64 -Ld:/Compiler/gcc-4.9.3/local330/lib -LC:/Users/YSU/R-33~1.1/bin/x64 -lR
2, test FORTRAN compiler
c:/Rtools/mingw_64/bin/gfortran -O2 -mtune=core2 -c R_FORTRAN_Test.f -o R_FORTRAN_Test.o
c:/Rtools/mingw_64/bin/gcc -shared -s -static-libgcc -o R_FORTRAN_Test.dll tmp.def R_FORTRAN_Test.o -Ld:/Compiler/gcc-4.9.3/local330/lib/x64 -Ld:/Compiler/gcc-4.9.3/local330/lib -lgfortran -lm -lquadmath -LC:/Users/YSU/R-33~1.1/bin/x64 -lR