Accept build dir and avoid dup run in env script

env.vortex-prebuilt.sh

@@ -1,4 +1,23 @@
-BUILDDIR=$HOME/build
+if [ $# -lt 1 ]
+then
+    echo "Usage: source env.vortex-prebuilt.sh build-dir"
+    return
+fi
+
+if [ -n "$VORTEX_ENV" ]
+then
+    echo "VORTEX_ENV already set. Exiting."
+    return
+fi
+
+BUILDDIR=$1
+
+if ! [ -d "$BUILDDIR/vortex-toolchain-prebuilt" ]
+then
+    echo "error: $BUILDDIR/vortex-toolchain-prebuilt does not exist."
+    return 1
+fi
+
 export VORTEX_ENV="vortex-prebuilt"
 export LLVM_PREFIX=$BUILDDIR/vortex-toolchain-prebuilt/llvm-riscv/
 export POCL_CC_PATH=$BUILDDIR/vortex-toolchain-prebuilt/pocl/compiler