implement a go-generate command for cgoflags

[sbinet]

finding the right combination of compilation flags for locating headers and linking against the right libclang is tricky and not user-friendly as (mainly) there is no pkg-config support for neither LLVM nor CLang.

there is a llvm-config program that mimics what pkg-config can do, but only for LLVM libraries, not libclang.
On top of that, llvm-config may not have been installed, or with a slightly different name, depending on the Linux distribution or OS.

to ease the burden on our brave users, we should probably have either a go run ./config-cgo.go program or a go generate step to perform the following drudgery:

• find llvm-config and extract/infer the correct CFLAGS and LDFLAGS (handling the possible naming variations of libclang.so[.MAJOR[.MINOR]]
• if no llvm-config could be found, try to look into canonical locations for the given libclang version, depending on the Linux distribution (or OS.)
• then generate a cgoflags_gen.go file with the correct // #cgo directives embedded and hard-coded.

[zimmski]

A general question, do we need repositories that are go-getable? I mean at the moment we say that users should use CGO_LDFLAGS="-Lllvm-config --libdir" go get -u github.com/go-clang/v3.7/... That is go-getable if libclang exists. I am asking because "go generate" (or any other code generation) means that the user has to do additional steps.