Address OCL Clang build configuration issues

Follows up on commit 8d289a8.

Author: Artem Gindinson

scripts/buildIGC.sh

@@ -100,7 +100,7 @@ cd workspace/igc
 echo "[Build Status] IGC commit hash below:"
 echo "[Build Status] All necessary repository Ready"
 
-CONFIG_VARS="-DIGC_OPTION__LLVM_MODE=Prebuilds -DIGC_OPTION__LLVM_PREFERRED_VERSION=$LLVM_VERSION_PREFERRED -DCCLANG_BUILD_PREBUILDS"
+CONFIG_VARS="-DIGC_OPTION__LLVM_MODE=Prebuilds -DIGC_OPTION__LLVM_PREFERRED_VERSION=$LLVM_VERSION_PREFERRED -DCCLANG_BUILD_PREBUILDS=ON"
 case $COMPILER in
     "clang")
         CONFIG_VARS="$CONFIG_VARS -DCMAKE_C_COMPILER=clang-$LLVM_VERSION -DCMAKE_CXX_COMPILER=clang++-$LLVM_VERSION"