It worked! I can't believe it!
Thank you VERY MUCH, jorrit!

Through "System"/"Advanced"/"Environment Variables"
The variable is CRYSTAL(all caps), value is the same directory I've compiled CS.(C:/CS, for example).
It is set as a System Variable, not as a User Variable. I've tried doing the opposite as well, and I also have tried adding it to both User and System variables, but it didn't work in any of the previous ways.
Thanks for the attention,

Hello there, I am a hobbyist 3d modeler and I thought about trying to make a game. I've chosen Crystal Space/Entity Layer as the graphics/game engines, and have compiled Crystal Space successfully. But, when I went to compile CEL, it simply didn't work...
It gives me the following error message during the ./configure process:
"checking if Crystal Space SDK is usable... no
configure: error:
*** Crystal Space could not be found or was unusable. The latest version is
*** always available from http://www.crystalspace3d.org/
*** Also, be sure that you have either installed Crystal Space or set the
*** CRYSTAL environment variable properly."

--The troubleshooting I've tried so far:
1.) I HAVE set the CRYSTAL environment variable
2.) I've tried to rename the folder(changing the EV accordingly)
3.) I've recompiled CS. Countless times.
4.) Started all the libs/MinGW/MSYS installing and CS compiling from scratch.

Neither worked. Unless I've done them wrong (which I doubt, because I've followed the tutorials and have already been able to compile/install CS/CEL before in another computer), I have NO idea what else to do.

I am using Windows as the OS and MinGW/MSYS to compile. All of the libs/CS/CEL versions are the latest stable ones.

PS.:Sorry about the bad grammar... English is not my native language. Just hope you guys can understand what I mean.

I just can't compile CEL. It keeps returning the message:
"checking if Crystal Space SDK is usable... no
configure: error:
*** Crystal Space could not be found or was unusable. The latest version is
*** always available from http://www.crystalspace3d.org/
*** Also, be sure that you have either installed Crystal Space or set the
*** CRYSTAL environment variable properly."
The troubleshooting I've tried so far:
1.) I HAVE set the CRYSTAL environment variable
2.) I've tried to rename the folder(changing the EV accordingly)
3.) I've recompiled CS.
Neither of these worked. I have just formatted my HD, and before formatting I used to have CS and CEL compiled, with no problems at all!
I am using Windows as the OS and MinGW to compile.

Edit: I have solved the problem by asking somewhere else in the forum. Thank you for your help.