Fix error locating llvm-ar during build on Linux

src/pal/tools/gen-buildsys-clang.sh

@@ -24,6 +24,39 @@ else
   buildtype=$2
 fi
 
+# Locate llvm
+# This can be a little complicated, because the common use-case of Ubuntu with
+# llvm-3.5 installed uses a rather unusual llvm installation with the version
+# number postfixed (i.e. llvm-ar-3.5), so we check for that first.
+desired_llvm_version=3.5
+locate_llvm_exec() {
+  if which $1-$desired_llvm_version > /dev/null 2>&1
+  then
+    echo $(which $1-$desired_llvm_version)
+  elif which $1 > /dev/null 2>&1
+  then
+    echo $(which $1)
+  else
+    exit 1
+  fi
+}
+llvm_ar=$(locate_llvm_exec llvm-ar)
+[[ $? -eq 0 ]] || { echo "Unable to locate llvm-ar"; exit 1; }
+llvm_link=$(locate_llvm_exec llvm-link)
+[[ $? -eq 0 ]] || { echo "Unable to locate llvm-link"; exit 1; }
+llvm_nm=$(locate_llvm_exec llvm-nm)
+[[ $? -eq 0 ]] || { echo "Unable to locate llvm-nm"; exit 1; }
+llvm_objdump=$(locate_llvm_exec llvm-objdump)
+[[ $? -eq 0 ]] || { echo "Unable to locate llvm-objdump"; exit 1; }
+llvm_ranlib=$(locate_llvm_exec llvm-ranlib)
+[[ $? -eq 0 ]] || { echo "Unable to locate llvm-ranlib"; exit 1; }
 
-cmake -DCMAKE_USER_MAKE_RULES_OVERRIDE=$1/src/pal/tools/clang-compiler-override.txt -D_CMAKE_TOOLCHAIN_PREFIX=llvm- -D_CMAKE_TOOLCHAIN_SUFFIX=-3.5 -DCMAKE_BUILD_TYPE=$buildtype $1
+cmake -DCMAKE_USER_MAKE_RULES_OVERRIDE=$1/src/pal/tools/clang-compiler-override.txt \
+  -DCMAKE_AR=$llvm_ar \
+  -DCMAKE_LINKER=$llvm_link \
+  -DCMAKE_NM=$llvm_nm \
+  -DCMAKE_OBJDUMP=$llvm_objdump \
+  -DCMAKE_RANLIB=$llvm_ranlib \
+  -DCMAKE_BUILD_TYPE=$buildtype \
+  $1