At the moment specifying GCC_REQ+=7 will fail with the rather cryptic "No USE_PKGSRC_GCC_RUNTIME support for gcc-7" if the package needs shared libraries (but work if shared libraries are not needed).
What do people think about making GCC_REQ+=7 a synonym for GCC_REQ+=7.0. Otherwise it probably makes sense to tweak
gcc.mk to explicitly error out with a more helpful message in this case :)
David