Check if custom clang version and release version is set before building libfuzzer.

Test: `make -j48 LLVM_PREBUILTS_VERSION=clang-stable LLVM_RELEASE_VERSION=5.0`

Change-Id: I8ddf353f8ec3ec98d2526379e2f20f4b177f7b28
(cherry picked from commit 6bf99fee15091f213a253ce9f07ba5f06750cf78)
3 files changed