Problems with paths

Hi all.
I have a problem to know where is the application path in windows and linux. As you know in windows, when you install your application you put all files in the same path (if you want, this is what i do):
c:/program files/myapp/execfile.exe
c:/program files/myapp/datafile.dat
and when I run my program (execfile.exe) I use something like this myapppath=app.applicationdirpath() to know where the program have been installed and call the data file mydatafile=myapppath+"/datafile.dat". This works fine.
But I have a problem in linux. When I copy the application, p.e., /home/david/apppath/, do a make to compile the application, but if i do a "make install", the exec file is copied to the /usr/local/bin/ directory (for example), and when I run the program from anywhere, the application cannot find the data file. I know that this is due to the appfile is in a directory (/usr/local/bin or other) and the data file is in other directory (for example /home/david/apppath).
What can I do?
Is there a way to know in linux where is the application and where have been installed the application? (if i use applicationdirpath() function i get the path where have been executed the program)

Thanks in advanced and I hope you understand my problem.