[MonoDevelop] Monodevelop just stopped recognizing some of my functions for no discernable reason

[That One Guy...]

This is a really weird problem i'm encountering, and i don't know how to deal
with it, so i was hoping to go here and maybe get some answers...

first, some background on the situation: 

A long time ago, when i was first forced to learn some basic programming
(c++) for my degree, i created this crappy little game along the lines of an
old text-based dungeon crawl, played through the terminal, to help me
remember things for my class.  a much shorter while ago (around october), i
decided to pick the game back up, and revamp it to be a little less like a
crappy beginner's programming experiment, and more like an actual game. 
I've been working on it on and off for a while, and i stopped working on it
entirely for about a month while i dealt with finals and remodeling the
house.  Now, i've got a little spare time, and i wanted to continue my work,
because i got some new ideas.  The last time i worked on the project it was
compiling correctly, and everything was working properly.  The problem i'm
having is that i started up MonoDevelop today, loaded my project, and out of
habit i went ahead and tried to build the project before i started working,
to make sure there were no errors.  unfortunately, this time i was met with
several un-explainable errors that i've never seen before.

The actual problem:
I've been using some functions i read about online to do some special tasks
for me in my program, such as make all the letters from the user input
lowercase to ease processing them, and a system call to the terminal to
blank the window after displaying some large ASCII pictures, or when leaving
the game menu.  and all of them were the ones that stopped working.  

I was wondering if these are supposed to be globally supported functions, or
if i'm missing an include somewhere.

heres my includes:
#include <iostream>

#include <string>
#include <cstdio>
#include <fstream>

and the functions that stopped working:
	cout << system("clear") << endl;	//send the 'clear' command to the
terminal, to blank the window

transform(input_root.begin(),input_root.end(),input_root.begin(),(int(*)(int))tolower);
//turn the user input string to all lowercase letters
	exit(0);	//a function i read about online that was supposed to make the
program exit safely when triggered

all of these were working properly up until today.  now, they're all giving
me the 'not declared in this scope' error... how can i fix it?

Thanks!
Andrew
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