When doing cross-compliation (for runc), the binaries are put into a
different directory and we need to support using those alternative build
directories as sources of installation.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Outputting .symver entries for staticlib builds leads to "multiple
definition" errors when linking statically, so we need to only output
.symver (and our version script) when building a cdylib.

Unfortunately, Rust provides no built-in mechanism to do this nicely.
There is no way to get the crate type using #[cfg(...)] and even an
explicit --cfg on the command-line does not show up in build.rs. The
only real "Rust-y" solution would be to create a separate feature for
cdylib but that is really ugly.

The solution I went with was passing an environment variable during the
build (LIBPATHRS_CAPI_BUILDMODE) which acts as a proxy for --crate-type
(in the hopes that this might be supported properly in the future).
build.rs then configures the #[cfg(cdylib)] used to control the output
of .symver and only outputs a version script. We even get to use
cargo:rerun-if-env-changed to make sure that the build script is
actually rebuilt for every crate type change. An unset
LIBPATHRS_CAPI_BUILDMODE (such as during tests) is treated like
staticlib.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

There were two separate issues when building on i386:

 * We used C.ulong rather than C.size_t internally when calling into
   readlink-related libpathrs APIs, which are different widths on i386.
   The solution is to just use C.size_t, which we should've used in the
   first place.

 * There was a comparison between int and 1<<31, which would overflow on
   i386. The solution is to cast to uint64, though this happens to cause
   a spurious G115 lint error from gosec which we need to mask.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>