updated install.html for info with xsm install error

1. Updated and removed the "lib6-dev" package installation command as there is no such package, the closest is "libc6-dev" package which is installed by default in most systems.
2. Install "libfl-dev" for Linux Mint (needed for flex), etc or any system which had their this particular package removed by subsequent updates of the OS. I found that in vanilla Linux Mint (powered up in VM) the "libfl-dev" package was present by default but was not present in my hard-disk installed and updated Linux Mint. So I replaced the "-ll" with "-lfl" (using the "fl" library) in the "cc" line of the "Makefile" present in xsm_dev.
3. Using any editor vi or nano, added  "#!/usr/bin/env bash" line as first line to the executable "xsm" file in xsm_expl folder so that regardless of which shell the user is using the executable "xsm" will always be run on the "bash" shell as how the "xsm" was natively written to run.

Author: faruk13

install.html

@@ -103,12 +103,23 @@ <h5> Step 2 :</h5>
             You may get some warnings and they can be ignored. If you get any <b>fatal error</b>,
             then install the following dependencies and try running "make" again : <br>
             <b>sudo apt-get install libreadline-dev</b> <br>
-            <b>sudo apt-get install lib6-dev</b> <br>
+            <b>sudo apt-get install libc6-dev</b> <br>
             If any other dependencies are missing (this depends on your system configuration), you have to install the missing dependencies
             and run "make" again.
+	    (Optional) While running "./xsm" after Step 2 if you get this error:<br>
+		  <i>/usr/bin/ld: cannot find -ll
+		  collect2: error: ld returned 1 exit status</i>
+		  Then you need to install:<br>
+		  <b>sudo apt-get install libfl-dev</b> <br>
+		  and edit the <b>Makefile</b> of <b>xsm_dev</b> folder, to proceed 
+		  find the line where "-ll" is used as option and update it to "-lfl" to use the "flex" library we installed above.
+	    Now you can run "make" again after navigating into the folder <b>xsm_expl</b> through the terminal.
           </p></li>
           <li id="otis">2. Type "cd ../xfs-interface/" and type "./init".</li>
-        </ul>
+	  <li id="otis">3.(Optional) Add this line <b>#!/usr/bin/env bash</b> as first line to the <b>xsm</b> file in <b>xsm_expl</b> folder.</li>
+          This step is for those who don't have <b>bash</b> or <b>sh</b> as their "default shell" and therefore may be using other 
+	  customizable shells like <b>zsh</b>, etc as their default shell. You can check your default shell by using <b>echo $SHELL</b> in terminal.
+	</ul>
     </p>
     <p>The usage instructions for the XSM simulator can be found <a href="xsmusagespec.html" target="_blank"> here</a>.</p>