It reduces the number of synonym variables in the code.
Makes it easier to grok and grep.
- replace `CURL_SOURCE_DIR`
with `PROJECT_SOURCE_DIR`.
- replace `CURL_BINARY_DIR`
with `PROJECT_BINARY_DIR` or `CMAKE_CURRENT_BINARY_DIR`.
- replace a single use of `CMAKE_BINARY_DIR`
with `PROJECT_BINARY_DIR`.
- replace `CMAKE_CURRENT_*_DIR`
with `PROJECT_*_DIR` where it makes the code more uniform.
- quote an argument (formatting).
Closes#15331
9126eb5a8a