Refactor installation of C API and features supported

[alexcrichton]

This commit overhauls and refactors the management of the building of
the C API. Instead of this being script-based it's now done entirely
through CMake and CMake is the primary focus for building the C API. For
example building the C API release artifacts is now done through CMake
instead of through a shell script's cargo build and manually moving
artifacts.

The benefits that this brings are:

• The C API now properly hides symbols in its header files that weren't
  enabled at build time. This is done through a new build-time generated
  conf.h templated on a conf.h.in file in the source tree.

• The C API's project now supports enabling/disabling Cargo features to
  have finer-grained support over what's included (plus auto-management
  of the header file).

• Building the C API and managing it is now exclusively done through
  CMake. For example invoking doxygen now lives in CMake, installation
  lives there, etc.

The CMakeLists.txt file for the C API is overhauled in the process of
doing this. The build now primarily matches on the Rust target being
built rather than the host target. Additionally installation will now
install both the static and shared libraries instead of just one.
Additionally during this refactoring various bits and pieces of
Android-specific code were all removed. Management of the C toolchain
feels best left in scope of the caller (e.g. configuring CC_* env vars
and such) rather than here.

[alexcrichton]

@rockwotj would you be able to give this a once-over perhaps? If not no worries, but you probably know more about CMake than I!

[dundargoc]

Nitpick: arguably cleaner

Suggested change

	if(${default})
	if(${WASMTIME_DISABLE_ALL_FEATURES})
	set(feature_default OFF)
	else()
	set(feature_default ON)
	endif()
	else()
	set(feature_default OFF)
	endif()
	set(feature_default "${default} AND NOT ${WASMTIME_DISABLE_ALL_FEATURES}")

[alexcrichton]

I don't think I fully understand cmake evaluation contexts and strings and what's a variable when, but when I use your suggestion it looks like it sets feature_default to the string ON AND NOT OFF and I'm not sure why but the default build has all features off.

I'm also not sure if I'm supposed to do something like if(${${cmake_name}}) below instead of just if(${cmake_name}) (sorry I'm very new to cmake)

[dundargoc]

Actually I retried it and my suggestion was wrong. My bad, I should've investigated better beforehand. Ignore my comment, your method is the way to go.

[alexcrichton]

Ah ok makes sense, and no worries! Thanks for taking a look regardless!

[rockwotj]

Very nice! This is a great change. Just a few small things

[rockwotj]

While I am thinking about it - the c compiler should be set as an environment variable for the cc crate. There are a number of ways to set it in cmake that don't involve the environment variables that the cc crate uses

[alexcrichton]

True! I'm a bit wary to do that though since cmake's cross-compilation defaults are not always as good as the cc crate's auto-detection. Do you know if there's a way to only pass the C compiler env vars and such if explicitly configured in cmake?

[alexcrichton]

Thanks @dundargoc and @rockwotj!