Help me making a small change in a C++ program

What you're trying to do isn't possible. #define is a preprocessor command, executed at compile time. Any instances of USE_KCHMVIEWER get replaced in the code, before it is compiled, with the value of USE_KCHMVIEWER. So there isn't a way to change such constants at run-time, and "undefining" them doesn't make any sense.

What you need to do is modify the code to remove USE_KCHMVIEWER from the relevant decision(s). Probably, it should be removed entirely and replaced with a variable. I'm not familiar with VirtualBox, so I don't know how difficult this is. But the type you would want to use is bool.

So, you're telling me that C++, a powerful programming language that's powering operating systems and state-of-the-art games, isn't powerful enough to define a simple conditional statement? Even a shell script (dash) can do this:

#!/bin/sh

which kchmviewer > /dev/null

if [ $? = 0 ]
then	# "which kchmviewer" exit code 0
	USE_KCHMVIEWER=1
else	# "which kchmviewer" exit code 1
	USE_KCHMVIEWER=
fi

if [ "$USE_KCHMVIEWER" = '' ]
then
	echo kchmviewer IS NOT installed.
else
	echo kchmviewer IS installed.
fi

USE_KCHMVIEWER is not used in the code, however, I would like to replace VBOX_OSE with it in the mentioned cases.

What do you mean by that USE_KCHMVIEWER isn't a variable? I understand that VBOX_OSE (some of whose occurrences I want to replace with USE_KCHMVIEWER) is a compile-time option. Now, I just want to create a code that could define this at run-time instead of compile-time, and then put it somewhere into the codebase. The expected result should be that whenever I access the documentation inside the GUI, either the CHM or PDF file will be opened based on what document viewer is available at the moment (kind of a OSE/non-OSE hybrid*).

So you're basically saying that variables (or whatever USE_KCHMVIEWER is) used in preprocessor statements (the lines with a hashtag) cannot be changed/redefined once the program is compiled because they are replaced by their value (i.e. they are hardcoded). What would be the preferred way to work around this (i.e. what should I use instead of # ifdef USE_KCHMVIEWER, and # if !defined USE_KCHMVIEWER)?

* https://forums.virtualbox.org/viewtopic.php?f=10&t=73979#p342127

I replaced #define with bool as suggested (note: why this doesn't work?)

if (system("which kchmviewer > /dev/null") == 0)
{
    // "which kchmviewer" exit code 0
    bool useKchmviewer = true;
}
else
{
    // "which kchmviewer" exit code 1
    bool useKchmviewer = false;
}

but the compilation was terminated with the following message (screenshot):

warning: unused variable ‘useKchmviewer’ [-Wunused-variable]

Based on what I read on the Internet (I need to declare a variable first, then I can change the value later), I tried a different approach (note: while this seems to work, I would also like to know how to do this, if possible, using an if…else statement as pointed out above):

bool useKchmviewer = false;

if (system("which kchmviewer > /dev/null") == 0)
{
    // "which kchmviewer" exit code 0
    useKchmviewer = true;
}

This worked, however once I tried to use this with

if (!useKchmviewer)
{
    const QString name = "UserManual";
    const QString suffix = "pdf";
}
else
{
    const QString name = "VirtualBox";
    const QString suffix = "chm";
}

the compiler was unhappy again (screenshot). Based on the answer from Stack Overflow, I altered this code too (note: why is this necessary?):

QString name = "UserManual";
QString suffix = "pdf";

if (useKchmviewer)
{
    name = "VirtualBox";
    suffix = "chm";
}

After this, the compilation finished successfully and the changes worked as expected. However, I still need to adapt other parts of the code so this is only a partial success.

P.S.: I understand how frustrating can be to help a person that doesn't have any knowledge in the topic he/she's expecting to get help in, but please bear with me.

warning: unused variable ‘useKchmviewer’ [-Wunused-variable]

That is not an error. Only a warning. The code was compiling successfully. The warning is telling you that the Boolean you defined is not used: you write in the variable "useKchmviewer" but never read from it.

Finally, I'm in the stage where my changes are working properly. There's one important thing left to do before they will be usable. I would like to know where should I place

bool useKchmviewer = false;

if (system("which kchmviewer > /dev/null") == 0)
{
    // "which kchmviewer" exit code 0
    useKchmviewer = true;
}

in the program codebase so I can check for variable useKchmviewer from wherever I want, specifically VirtualBox-5.0.6/src/VBox/Frontends/VirtualBox/src/globals/UIMessageCenter.cpp and VBoxGlobal.cpp. How can I do this?