Remove CMake requirement for builds

[camshaft]

Problem:

CMake is not a ubiquitous build tool; for example it is not automatically installed with the build-essential. By requiring CMake, aws-lc-rs puts additional build requirements on environments, which will potentially hinder adoption.

Solution

Instead, the library should have a build system that uses more ubiquitous tools. I think make would be a step in the right direction, but I think ideally the only requirement should be a C compiler and assembler. This is the approach that was taken in s2n-tls-sys which makes it quite easy to build.

[justsmth]

Related to #152

[justsmth]

As of release v1.7.0, CMake is no longer required to build aws-lc-rs for Linux/MacOS on x86-64/aarch64. For these platforms, only the minimal Rust & C toolchains should be required for the build to succeed.

I'm going to close this issue now. Feel free to open another issue to request extending support for our no-cmake build to other platforms (e.g., Windows).