configure.ac: rework llvm libs handling for 3.9+

Earlier versions need different quirks, but as of LLVM 3.9 llvm-config
provides --link-shared/link-static toggles.

The output of which seems to be reliable - looking at LLVM 3.9, 4.0 and
5.0.

Note that there are earlier code will be used for pre LLVM 3.9 and is
unchanged.

This effectively fixes LLVM static linking, while providing a clearer
and more robust solution for future versions.

Mildly interesting side notes:

 - build-mode (introduced with 3.8) was buggy with 3.8
It shows "static" when build with -DLLVM_LINK_LLVM_DYLIB=ON, yet it was
consistent with --libs. The latter shows the static libraries.

 - libnames and libfiles are broken with LVM 3.9
The library prefix and extension is printed twice liblibLLVM-3.9.so.so

v2: Invoke llvm-config twice, instead of using sed, to combine the two
lines into one (Tobias)

Cc: [email protected]
Cc: Dieter Nützel <[email protected]>
Cc: Michel Dänzer <[email protected]>
Signed-off-by: Emil Velikov <[email protected]>
Reviewed-by: Tobias Droste <[email protected]>

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