Support aws-lc-sys on x86_64-unknown-freebsd without cmake

[LHolten]

At work we would like to build aws-lc-sys without cmake for the x86_64-unknown-freebsd target.
From what I understand this is supported on a few platforms already with pre-generated bingen output.

I can probably make a PR if that is desired, depending on how difficult this is.

Problem:

Reproduce the problem with cargo build --target x86_64-unknown-freebsd on a system without cmake.
Or run AWS_LC_SYS_CMAKE_BUILDER=0 cargo build --target x86_64-unknown-freebsd.

   Compiling aws-lc-sys v0.28.0 (/home/lucasholten/opensource/aws-lc-rs/aws-lc-sys)
error: failed to run custom build command for `aws-lc-sys v0.28.0 (/home/lucasholten/opensource/aws-lc-rs/aws-lc-sys)`

Caused by:
  process didn't exit successfully: `/home/lucasholten/opensource/aws-lc-rs/target/debug/build/aws-lc-sys-b310cb966a2d0e6f/build-script-main` (exit status: 101)
  --- stdout
  cargo:rerun-if-env-changed=AWS_LC_SYS_NO_PREFIX
  cargo:rerun-if-env-changed=AWS_LC_SYS_PREGENERATING_BINDINGS
  cargo:rerun-if-env-changed=AWS_LC_SYS_EXTERNAL_BINDGEN
  cargo:rerun-if-env-changed=AWS_LC_SYS_NO_ASM
  cargo:rerun-if-env-changed=AWS_LC_SYS_CFLAGS
  cargo:rerun-if-env-changed=AWS_LC_SYS_PREBUILT_NASM
  cargo:rerun-if-env-changed=AWS_LC_SYS_C_STD
  cargo:rerun-if-env-changed=AWS_LC_SYS_CMAKE_BUILDER
  cargo:rerun-if-env-changed=AWS_LC_SYS_NO_PREGENERATED_SRC
  cargo:rerun-if-env-changed=AWS_LC_SYS_STATIC
  cargo:rerun-if-env-changed=CMAKE

  --- stderr
  Missing dependency: cmake

  thread 'main' panicked at aws-lc-sys/builder/main.rs:382:40:
  called `Result::unwrap()` on an `Err` value: "Required build dependency is missing. Halting build."
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace