Fix clang compiler detection on OS X

check-tools.pl was hard-wired to expect the Apple compiler version string which
doesn't make much sense, especially for a LLVM project.

The correctness and value of these compiler version checks still isn't clear
following this build fix but they are retained for now.

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@202029 91177308-0d34-0410-b5e6-96231b3b80d8
Fix linking and get the build working on OS X

No version of clang installed by XCode, ToT or otherwise recognizes the flag
'-no-intel-extensions' and more recent versions error out on it.

Prospectively conditionalize the flag behind "icc" to fix the mainline build
with clang.

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@202030 91177308-0d34-0410-b5e6-96231b3b80d8