[Beignet] [PATCH] Make FindLLVM.cmake work with empty "system-libs"

[Steven Newbury]

On Gentoo we sanitise and fix up llvm-config to work with shared
libraries and to provide correct output for @world to build against.
 (See https://bugs.gentoo.org/show_bug.cgi?id=565358) Part of this is
we wipe "system-libs" since it's inappropriate to link to
unnecessarily.

This has revealed the FindLLVM.cmake included with Beignet fails when
system-libs is empty.  Attached patch fixes this by checking for a
result before calling REGEX.
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