This uses a .PHONY rule to just run cargo every time.  There is no
addition to "make clean" yet.

The resulting librsvg_internals.so is trivially linked into the LIBADD
for the main librsvg library.

I don't know how to handle "cargo build --release" just for the final
version, while "cargo build" normally.  For the C stuff I just pass
CFLAGS by hand to autogen.sh.

This is not integrated into configure.ac yet!