gmp/mpfr: make sure host builds add source targets to HOST_SOURCE

Closes #1183.

When gmp/mpfr is needed for the host (E.G. when using an internal toolchain),
the host-lib{gmp,mpfr}-source targets weren't added to HOST_SOURCE, so
make source / external-deps didn't handle them.

Notice that we have the same issue with the new host package support,
there we should probably use HOST_<package>_DEPENDENCIES for -source
dependencies.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>