(math/R-classInt) Add compiler.mk buildlink

Without this change, I have following error with 9.99.82 host (amd64) which
has gcc-10. And buildlink for libgfortran was only made for gcc-10 side.
Adding this line makes both gcc-7 and gcc-10 buildlinked.

The error I had was: ( the line folded)
** libs
----------
gcc -shared -Wl,-R/usr/pkg/lib/R/lib -L/usr/pkg/lib/R/lib
  -L/usr/pkg/lib -L/usr/lib -Wl,-R/usr/lib -Wl,-R/usr/pkg/lib -pthrea d
  -L/usr/X11R7/lib -Wl,-R/usr/X11R7/lib -o classInt.so fish1.o init.o
  -L/usr/pkg/gcc7/lib/gcc/x86_64--netbsd/7.5.0 -L/usr/pkg/gcc7/lib
  -Wl,-R/usr/pkg/gcc7/x86_64--netbsd/lib/. -Wl,-R/usr/pkg/gcc7/lib/. -lgfortran
  -lm -lquadmath -lpthread -Wl,-R/usr/pkg/lib/R/lib -L/usr/pkg/lib/R/lib
  -lR
ld: cannot find -lgfortran
ld: cannot find -lquadmath
*** Error code 1
----------
Please correct me if my understanding (or correction) is wrong, sorry and tks


(mef)