Copy README.md/amalgamate_demo.cpp only if output dir isn't the same as ${SCRIPTPATH} (#1113)

+ use test's `-ef` file operator to check if ${SCRIPTPATH} and ${AMALGAMATE_OUTPUT_PATH} are not the same

singleheader/amalgamate.sh

@@ -127,8 +127,11 @@ echo "/* auto-generated on ${timestamp}. Do not edit! */" > ${AMAL_C}
     done
 } >> ${AMAL_C}
 
+# copy amalgamate_demo.cpp and README.md only if AMALGAMATE_OUTPUT_PATH is not the same as SCRIPTPATH
+if [ ! ${SCRIPTPATH} -ef ${AMALGAMATE_OUTPUT_PATH} ]; then
     cp -f "${SCRIPTPATH}/amalgamate_demo.cpp" "${DEMOCPP}"
     cp -f "${SCRIPTPATH}/README.md" "${README}"
+fi
 
 echo "Done with all files generation."