bug#25648: pcre_jit_stack not in centos6 pcre

[Karl Berry]

Trying to compile grep-2.28 on centos6.8 (32-bit, but I don't think that
matters), with the system compiler (gcc 4.4.47-17) and system pcre, I get:

pcresearch.c:53: error: expected specifier-qualifier-list before 
'pcre_jit_stack'

That line is:
  pcre_jit_stack *jit_stack;

And indeed, /usr/include/pcre.h does not contain anything about jit.
Evidently it is too old. I suggest that the configure test for pcre be
adjusted if you're going to require jit.

With --disable-perl-regexp, all went through. --thanks, karl.