When using a compiler to drive linking we add compiler flags from
`CMAKE_<LANG>_FLAGS` in case they affect the way the compiler invokes
the linker, but we don't add flags from other places that are meant only
for compiling sources.  Rather than calling the `AddLanguageFlags`
method (which is used to add flags for compiling sources) directly, add
an intermediate method that is used when adding the flags for linking.
This will give us a way to add language-specific compiler flags needed
when driving the linker in the same place on the command line as other
compiler flags go.