CMakeLists.txt: add env vars to target_include_directories

[a-andreyev]

Provided solution: https://github.com/a-andreyev/olm/commit/5421c05f86e6936ccfb381d263fe35aab0b230da

This makes possible to use Olm as an in-tree dependency too (not only as a system-side package). For example, for the libQtOlm project to organize CI builds for the libQuotient based projects (like Quaternion)

Would be happy to remove my fork and use matrix-org repo instead then 🙂

[uhoreg]

Merged: https://gitlab.matrix.org/matrix-org/olm/commit/aa0c9ab6b51d182cb78ab4cc75c81c0054765bde

Thanks