Missing LLVM Target Initialization APIs

[junlarsen]

Partially related to #932

In the llvm-c/Target.h header there are six macros which declare functions for target initialization. These are ran for each target to generate a set of APIs exposed to LLVM-C.

Javacpp is currently not properly read though this (probably because the llvm/Config/Targets.def file is created during build generation.

Fix

I'm not sure how we want to go about adding these APIs as they should only be available if the target is enabled. With the current build setup, that would be one target, but a different one for each platform we build the binaries for.

1. Creating a stub file (similar to LLVM full optimization #869) in the resources directory and add it to the preset for ALL targets
   This solution means that the initialization API for every target will be "available" regardless of which target the user's binary has enabled. This is probably not wanted behavior because calling into a function which doesn't actually exist will crash the JVM.

2. Parse the llvm-c/Target.h file after build generator step, assuming the Javacpp parser is able to perform macro expansion.
   This requires the parser to go over the files after the build has been generated.

I have created an example showing how this could be done if we go with option 1 here

[saudet]

Is there a reason why the LLVMInitializeAllTargets(), etc functions not enough?

[junlarsen]

Generally yeah, they're more than enough. I just noticed they were missing from the javacpp output and wanted to raise awareness. There would be some unnecessary initalization if one doesn't use all the targets, but it's a very minor thing.

[saudet]

The problem isn't macro expansion, it's because JavaCPP doesn't process #include directives, the idea being that users best know in what order they should be parsed, but it doesn't work when the same file is included multiple times in another file like this with different macro values.

[saudet]

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