modify install script to accept alternate install prefix from environment

.gitignore

@@ -24,3 +24,4 @@ autom4te.cache
 *.swp
 hash/
 standalone/
+build/

install

@@ -8,8 +8,12 @@ else
     FLAG_DIR=$1
 fi
 
-# Change this to match your destination directory
-prefix=/home/groups/flag/hash/
+# Set default install location if prefix is not in environment
+if [ -z ${prefix+x} ]
+then
+    prefix=/home/groups/flag/hash/
+fi
+echo "Installing FLAG to ${prefix}"
 
 # Add prefix to PATH
 if [[ "$PATH" == ?(*:)"$prefix/bin"?(:*) ]]