Only use libdl when it is available

Summary: On BSDs, there is no `libdl.so`, and functions like `dlopen`
are implemented in the main C library instead.  Use the `CMAKE_DL_LIBS`
variable instead of hardcoding a dependency on the `dl` library.

Reviewers: grokos, joerg, emaste

Reviewed By: emaste

Subscribers: jlpeyton, mgorny, openmp-commits

Differential Revision: https://reviews.llvm.org/D34632

llvm-svn: 306319