Makes the LLVM triple of the current platform available to Lean code
towards a solution for #2754.
Defaults to the empty string if the compiler is not clang, which can
introduce some divergence between CI and local builds but should not be
noticeable in most cases and is not really possible to avoid.
This is an additional safety net on top of #2749: it protects users that
circumvent the build system (e.g. with `lake env`) as well as obviates
the need for TOCTOU-like race condition checks in the build system.
The check is activated by `CHECK_OLEAN_VERSION=ON`, which now defaults
to `OFF` as the sensible default for local development. When activated,
`USE_GITHASH=ON` is also force-enabled for stage 0 in order to make sure
that stage 1 can load its own core library.
1) When git sha1 is available:
Lean (version 0.2.0, commit 4a119a0424, Release)
2) When git sha1 is not available, but there is bin/package_version file
Lean (version 0.2.0, package 0.2.0~ubuntu1~12.04, Release)
3) Git sha1 is not available, bin/package_version does not exist.
Lean (version 0.2.0, Release)
Close#229
