odhcp6c: update cmake file

odhcp6c: update cmake file

Now that the minimum cmake version has been bumped, the cmake file can be
modernized a bit. Although it might look like a lot of changes, most of them
are quite straightforward.

Every library is located in a consistent manner (allowing command-line
overrides of library locations, useful for local development, and also makes it
trivial to do static linking, if desired).

The compiler flags have been broken up to have one per line (making it easy in
the future to add/remove flags, which would create a simple one-line diff).

Although it might look like this bumps the C standard level, cmake is actually
smart enough to pick an earlier version if C11 isn't supported by the compiler
(quite unlikely on any versions of gcc currently in use in OpenWrt and even on
old distros).

Signed-off-by: Álvaro Fernández Rojas <[email protected]>