runtime Makefile: Do not resolve variables when writing a dependency file

[paulcadman]

This is an attempt to fix a confusing situation where a user:

1. Builds the runtime with clang that does not support wasm32-wasi
2. Attempts to rebuild the runtime by passing the CC parameter pointing to another installation of clang that does support wasm32-wasi.

In step 1. the runtime Makefile generates dependencies files which contain the resolved value of $(CC). When the user passes the correct CC variable to the Makefile in step 2., the dependencies files are not regenerated, the old value of CC is used in the build and the build continues to fail.

In this PR we change the dependency file generation so that the variables like $(CC) are written into the dependency file verbatim. They are resolved when they are run in step 2. using the new value of the CC parameter.

• Closes The steps in the "Build from Source" documentation always fail if the runtime is not cleaned after building with a non-wasi compatible clang #2537