[RFC] Fix issue 1569: Build fails in rpmbuild environment

Detect if there's a prefix to -D_FORTIFY_SOURCE defined in CFLAGS and propagate that prefix to the flags added to redefine _FORTIFY_SOURCE.  Addresses issue [#1569](https://github.com/neovim/neovim/issues/1569)
Why not just use `message(STATUS "Detected...")`?