Allow removing deprecated virtual method from own build

Virtual methods and other things with ABI-sideeffects beyond own symbol
can be wrapped with the *_BUILD_DEPRECATED_SINCE(x,y) in the headers,
to enable builds without those. These macros are only influenced by
variables baked into the build of the library (i.e. the export header),
cannot be controlled from 3rd-party, so results in a stable ABI.

CCBUG: 423003
GIT_SILENT