clang build fix: don't check for gnu tools by default

gcc/g++ don't exist on modern OS X development environments so the failed
checks for gcc were causing the build to break even with 'make compiler=clang'.

(The rationale for the check refers to a file 'touch-test.c' which no longer
exists in the codebase.)

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@202027 91177308-0d34-0410-b5e6-96231b3b80d8