* Refactor installation of C API and features supported
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.
prtest:full
* Don't use `option` for optional strings
* Invert release build check
Also adjust some indentation
* Fix more indentation
* Remove no-longer-used variable
* Reduce duplication in feature macro
Alex Crichton
566669eed2