Use the library path in the environment when looking up clang library.

Use *LD_LIBRARY_PATH when configured to build against the systems libclang.
This patch makes the install script work even when libclang is in a custom path.

Signed-off-by: Ola Jeppsson <ola.jeppsson@gmail.com>
Merge pull request #93 from olajep/use-env-for-clang-lookup-try2

[Try 2] Use the library path in the environment when looking up clang library.