[cmake] Fix builtin usage of GLEW

[linev]

On Mac platform without installed GLEW one gets error by first invocation of cmake:

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
/Users/user/git/root/graf3d/gl/GLEW_INCLUDE_DIR
   used as include directory in directory /Users/user/git/root/graf3d/gl

Problem that setting cache variable is not enough to overwrite already existing variable
So set variables as well.

[bellenot]

Thanks Sergey! (just a small typo)

[github-actions]

Test Results

    20 files      20 suites   3d 14h 19m 30s ⏱️
 3 746 tests  3 745 ✅ 0 💤 1 ❌
73 177 runs  73 176 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 158ff5e.