That part of code in configure.ac look stupid, remove it.
If we _do_ try to run BookmarkFS on such an exotic platform with
non-zero null pointers, the breakage should be detected by the tests
(if it can run or even build at all).
There's only a few legacy releases of gcc and clang that
support -fmacro-prefix-map but not __FILE_NAME__.
Thus the originial workaround is not very beneficial.
Switching to a more portable `__FILE__ + offset` workaround
that works for any standard-compliant compiler, even TinyCC.
However, this workaround may produce binaries that leak
information about the build environment, especially when using
an exotic build path.
