Old versions of gcc cannot compile 4.14 since 4.14.113:
./include/asm-generic/fixmap.h:37: error: implicit declaration of function ‘__builtin_unreachable’
The stable commit that caused the problem is 82017e26e515 ("compiler.h: update definition of unreachable()") (upstream commit fe0640eb30b7). Reverting the commit fixes the problem.
Kernel 4.17 dropped support for older versions of gcc in upstream commit cafa0010cd51 ("Raise the minimum required gcc version to 4.6"). This was not backported to 4.14 since that would go against the stable kernel rules.
Upstream commit 815f0ddb346c ("include/linux/compiler*.h: make compiler-*.h mutually exclusive") was a fix for cafa0010cd51. This was not backported to 4.14.
Upstream commit fe0640eb30b7 ("compiler.h: update definition of unreachable()") was a fix for 815f0ddb346c. This is the commit that was backported to 4.14. But it only fixed a problem introduced in the other commits, and without those commits, it ends up introducing a problem instead of fixing one. So I recommend reverting that patch in 4.14, which will enable old gcc to compile 4.14 again. If I understand correctly, I believe that clang will still be able to compile 4.14 with the patch reverted, although I haven't tried to compile with clang.
The problematic commit is not present in 4.9.x, 4.4.x, 3.18.x, or 3.16.x.
Tony Battersby Cybernetics