configure: quote variables

These should probably be quoted.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>

Author: cardoe

configure

@@ -1474,7 +1474,7 @@ fi
 step_msg "configuring submodules"
 
 # Have to be in the top of src directory for this
-if [ -z $CFG_DISABLE_MANAGE_SUBMODULES ] && [ -z $CFG_ENABLE_RUSTBUILD ]
+if [ -z "$CFG_DISABLE_MANAGE_SUBMODULES" ] && [ -z "$CFG_ENABLE_RUSTBUILD" ]
 then
     cd ${CFG_SRC_DIR}
 
@@ -1550,7 +1550,7 @@ do
     then
         msg "not configuring LLVM, rustbuild in use"
         do_reconfigure=0
-    elif [ -z $CFG_LLVM_ROOT ]
+    elif [ -z "$CFG_LLVM_ROOT" ]
     then
         LLVM_BUILD_DIR=${CFG_BUILD_DIR}$t/llvm
         LLVM_INST_DIR=$LLVM_BUILD_DIR