fix headless-build on linux

in such cases, DISABLE_HEADLESS was unset and hence
the expansion -a -eq 0 resulted in a bash error:

bash: [: too many arguments

this is fixed now by using string-based comparison

http://code.google.com/p/phantomjs/issues/detail?id=197

deploy/build-helper.sh

@@ -150,7 +150,7 @@ patch_qt() {
         patch -p1 < ../qt48_enable_file_input_click.patch
 
         # Build in lighthose mode for an x-less build
-        if [ $QT_HEADLESS -eq 1 -a $DISABLE_HEADLESS -eq 0 ] ; then
+        if [[ "$QT_HEADLESS" == "1" && "$DISABLE_HEADLESS" != "1" ]] ; then
             echo "Building 4.8 in qpa headless mode"
             QT_CFG+=' -qpa '
         fi