Summary:
Commit 4f81ab38 has the test wrong.

clang doesn't support a -dumpversion option. By lucky coincidence
clang/gcc --version both place a version number at the same output location
when --verison is passed.

Example output (1st line only).

    $ clang --version
    clang version 3.9.1 (tags/RELEASE_391/final)

    $ gcc --version
    gcc (GCC) 6.4.1 20170727 (Red Hat 6.4.1-1)

During the test of the compiler we ensure that a minimum version is met
as Makefile doesn't support patterns.

Also xcode9 doesn't seem affected by https://github.com/facebook/rocksdb/issues/2672
and also doesn't have "clang" as the first part of its output so the
fix implemented here also is Apple clang friendly.

    $ clang --version
    Apple LLVM version 9.0.0 (clang-900.0.31)
Signed-off-by: NDaniel Black <daniel.black@au.ibm.com>
Closes https://github.com/facebook/rocksdb/pull/2699

Differential Revision: D5600818

Pulled By: yiwu-arbug

fbshipit-source-id: 3b0f2751becb53c1c35468bf29f3f828e7cf2c2a