Change build.sh basedir

Use different method to get the basedir in site/build/build.sh allowing
the script to work on Windows as well.

Take from:
https://stackoverflow.com/questions/59895/getting-the-source-directory-of-a-bash-script-from-within

site/build/build.sh

@@ -3,7 +3,7 @@
 # Put all available languages here, except "en". Separated by spaces
 TRANSLATIONS="ca da de el eo es fr hu it nb nl pt sv tr zh_tw"
 
-basedir="${0%/*}/.."
+basedir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )/.."
 cd "$basedir"
 mode=$1