PCRE JIT is not supported on most architectores: m68k, s390,
ia64 to name a few.
Example build failure is:
$ nix build -f. pkgsCross.s390.pcre
...
pcre_jit_compile.c:65:2: error: #error Unsupported architecture
The change shifts from --enable-jit[=yes] to --enable-jit=auto which
allows nixpkgs to enable jit automatically only on supported targets.