When cross-compiling, `cfg!(target_os)` and friends will be the
configuration for the host rather than the target, since the build
script runs on the host. Cargo provides the target's configuration via
environment variables, so use those.

If we fail because there seems to be something wrong with library names,
provide more output on what's expected and what we found. This should
make cross-compilation related errors easier to spot, including Windows
target-env mismatches as in #24 (which was user error but not easy to
determine).

Be smarter about target-env

See merge request !13

I'm not sure why I thought these didn't exist, since on examination
upstream it seems the AsmPrinter has been implemented from very early in
the RISC-V backend's life; assume the others were too.

orc2: Make struct fields public

See merge request !14

orc2: Derive standard traits for enum types

See merge request taricorp/llvm-sys.rs!16

Fixed LLVMOrcGenericIRModuleOperationFunction parameter order.

See merge request !19

Add LLVMBuildPtrDiff2

See merge request taricorp/llvm-sys.rs!21

Before this change, installing llvm-sys was sometimes failing with the
"Could not determine LLVM version from llvm-config" error, which was
occuring because of lack of straightforward error on non-existing
llvm-config binary call.

Fixes: #33

build: Return an error when the output of llvm-config is empty

Closes #33

See merge request taricorp/llvm-sys.rs!22

On Alpine Linux, old LLVM versions have binaries named llvm${major}-blah.

Support for Alpine Linux llvm-config naming

See merge request !24

This includes the following API changes:

* New BLAKE3 API:
  * Structs:
    * `llvm_blake3_chunk_state`
    * `llvm_blake3_hasher`.
  * Functions:
    * `llvm_blake3_version`
    * `llvm_blake3_hasher_init`
    * `llvm_blake3_hasher_init_keyed`
    * `llvm_blake3_hasher_init_derive_key`
    * `llvm_blake3_hasher_init_derive_key_raw`
    * `llvm_blake3_hasher_update`
    * `llvm_blake3_hasher_finalize`
    * `llvm_blake3_hasher_finalize_seek`
    * `llvm_blake3_hasher_reset`
* New functionns for interaction with opaque pointers:
  * `LLVMContextSetOpaquePointers`
  * `LLVMPointerTypeIsOpaque`
  * `LLVMPointerTypeInContext`
* Change of signature of `LLVMOpInfoCallback` function type:
  * New arguments `OpSize` and `InstSize`.
  * Removal of `Size` argument.
* New RMW atomic operations:
  * `LLVMAtomicRMWBinOpFMax`
  * `LLVMAtomicRMWBinOpFMin`
* New LLVM binary type:
  * `LLVMBinaryTypeOffload` - offloading fatbinary.
* New JIT symbol generic flag:
  * `LLVMJITSymbolGenericFlagsNone`
* Renaming of `LLVMJITCSymbolMapPair` to `LLVMOrcCSymbolMapPair`.
* New struct and type for JITDylib search order:
  * `LLVMOrcCJITDylibSearchOrderElement`
  * `LLVMOrcCJITDylibSearchOrder`
* New ORC function types:
  * `LLVMOrcDisposeCAPIDefinitionGeneratorFunction`
  * `LLVMOrcExecutionSessionLookupHandleResultFunction`
* New ORC functions:
  * `LLVMOrcExecutionSessionLookup`
  * `LLVMOrcLookupStateContinueLookup`
* Change of signature of `LLVMOrcCreateCustomCAPIDefinitionGenerator`:
  * New `Dispose` argument.
* Removal of `coroutines` header, which included functions:
  * `LLVMAddCoroEarlyPass`
  * `LLVMAddCoroSplitPass`
  * `LLVMAddCoroElidePass`
  * `LLVMAddCoroCleanupPass`
  * `LLVMPassManagerBuilderAddCoroutinePassesToExtensionPoints`
* Removal of IPO functions:
  * `LLVMAddArgumentPromotionPass`
  * `LLVMPassManagerBuilderPopulateLTOPassManager`
* Removal of one calar function:
  * `LLVMAddLoopUnswitchPass`
* New `LLVMGetCastOpcode` function to aid users of `LLVMBuildCast` in
  resolving the best cast operation given a source value and
  destination type. This function is a direct wrapper of
  `CastInst::getCastOpcode`.
* New `LLVMGetAggregateElement` function as a wrapper for
  `Constant::getAggregateElement`, which can be used to fetch an element
  of a constant struct, array or vector, independently of the underlying
  representation. The `LLVMGetElementAsConstant` function is deprecated
  in favor of the new function, which works on all constant aggregates,
  rather than only instances of `ConstantDataSequential`.
* Removal of the following functions for creating constant expressions,
  because the underlying constant expressions are nno longer supported.
  Instead, an instruction should be created using the `LLVMBuildXYZ`
  APIs, which will constant fold the operands if possible and create an
  instruction otherwise:
  * `LLVMConstExtractValue`
  * `LLVMConstInsertValue`
  * `LLVMConstUDiv`
  * `LLVMConstExactUDiv`
  * `LLVMConstSDiv`
  * `LLVMConstExactSDiv`
  * `LLVMConstURem`
  * `LLVMConstSRem`
  * `LLVMConstFAdd`
  * `LLVMConstFSub`
  * `LLVMConstFMul`
  * `LLVMConstFDiv`
  * `LLVMConstFRem`
* New `LLVMDeleteInstruction` function which allows deleting
  instructions that are not inserted into a basic block.

Signed-off-by: Michal Rostecki <vadorovsky@gmail.com>

In the C API that struct is public because it's embedded in the public
llvm_blake3_hasher struct, but Rust allows us to make that type private.

On Arch Linux, we get the string '/usr/lib/libzstd.so.1.5.2' in
llvm-config's output. The library name is just `zstd` so we need to trim
the shared object version from the name as well as the `lib` prefix.

This only changes handling of system libraries so LLVM 15 works on Arch
Linux.

The `ID` parameter was missing