Makefile hard coded to use clang

The Makefiles for `caption-inspector` appear to be all hard coded to use `clang`:

https://github.com/Comcast/caption-inspector/blob/476326f08a43ce38ecd1ea58b8910d7015e80cac/src/Makefile#L137-L143

This project builds fine on gcc 9.3 too.

One can use [the implicit variable](https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html) `$(CC)` to get whatever the user-preferred compiler (and its location) is. It also makes it easier to set up a CI build matrix with other compilers.