build: fix enforce unique symbols for recent clang version

build: fix enforce unique symbols for recent clang version

clang 6.0 and newer behave like gcc in regards for the FILE symbol, so
only the filename rather than the full path to the source file.

clang 3.8.1-24 (in our debian:stretch container) and 3.5.0-10
(in our debian:jessie container) do store the full path to the source
file in the FILE symbol.

Also we have commit 81ecb38b83 ("build: provide option to disambiguate
symbol names") which were using clang 5, and LLVM's commit f5040b9685a7
[1] ("Make .file directive to have basename only") which is part of
"llvmorg-6.0.0" tag but not "release/5.x" branch. Both suggest that
clang change of behavior happened with clang 6.0.

This means that we also need to check clang version to figure out
which command we need to use to redefine symbol.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
[1] https://github.com/llvm/llvm-project/commit/f5040b9685a760e584c576e9185295e54635d51e