configure: Check llvm-config --shared-mode

configure: Check llvm-config --shared-mode

https://bugs.llvm.org/show_bug.cgi?id=6823 still affects current LLVM.
llvm-config --libs only reports the single shared library if LLVM was
built with -DLLVM_LINK_LLVM_DYLIB=ON. llvm-config --shared-mode reports
"shared" in that case, "static" otherwise (even if LLVM was built with
-DLLVM_BUILD_LLVM_DYLIB=ON).

v2: Keep the LLVM < 4.0 test. (llvm-config --shared-mode is actually
    available since LLVM 3.8, but that would make the test too
    complicated :)

Fixes: 3d8da1f678e1 ("configure: Trust LLVM >= 4.0 llvm-config --libs
                      for shared libraries")
Bugzilla: https://bugs.freedesktop.org/102247
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>