Strip folly's own root from XLOG categories, not the outer project's

[gmarzot]

Problem

The comment above this property states the intent:

using only the relative portion of the source file name inside the folly repository

but the value is built from CMAKE_SOURCE_DIR / CMAKE_BINARY_DIR, which point at the top-level project. Those equal folly's own roots only when folly is the top-level build.

When folly is embedded — FetchContent, add_subdirectory, or any superproject — CMAKE_SOURCE_DIR is the consumer's root and is not a prefix of folly's source paths. xlogStripFilename() then strips nothing, and every folly category falls back to the full absolute __FILE__. Two consequences for embedded consumers:

• --logging=folly=DBG2 and every folly.* selector match nothing; only the root level works.
• category names leak absolute build paths, e.g. home.runner.work.myproject._deps.folly-src.folly.io.async.AsyncSocket.

Fix

Use CMAKE_CURRENT_SOURCE_DIR / CMAKE_CURRENT_BINARY_DIR. Evaluated in folly's own top-level CMakeLists.txt, these always resolve to folly's source and build roots regardless of how folly is embedded — and it's the same variable the surrounding set_property(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} …) call already uses to scope the property.

No behavior change when folly is the top-level project, since CMAKE_SOURCE_DIR == CMAKE_CURRENT_SOURCE_DIR in that case. The change only affects builds where the current behavior is already broken.

Test plan

• Embedded build (folly via FetchContent in a superproject): before, folly categories were the absolute __FILE__ and --logging=folly=DBG2 selected nothing; after, categories derive as folly.* and the selector scopes correctly.
• Top-level folly build: FOLLY_XLOG_STRIP_PREFIXES expands identically before and after.

Same class of fix as facebookexperimental/moxygen#207, which corrected the identical CMAKE_SOURCE_DIR assumption in moxygen's own use of this define.