Updated build_doc…sh to run cleanly in Xcode build

If the lexicon input filter isn't specified (as is the case in the
current Xcode project, the script quietly continues without it.

build_tools/build_documentation.sh

@@ -68,6 +68,12 @@ if test -z "$DOXYGENPATH"; then
     exit 0
 fi
 
+# Check we have the lexicon filter
+if test -z "$INPUT_FILTER"; then
+    echo >&2 "Lexicon filter is not available. Continuing without."
+    INPUTFILTER=''
+fi
+
 # Determine where our output should go
 if ! mkdir -p "${OUTPUTDIR}" ; then
     echo "Could not create output directory '${OUTPUTDIR}'"