[Cmake] Use CMAKE_PROJECT_NAME in src/config/CMakeLists.txt

Summary:
If someone wants to rename the project, they'd also need to change these variables.

Instead, we fix this by using CMAKE_PROJECT_NAME when building the config header.

Test Plan: ninja check && ninja check-functional

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D17178