BUG/MINOR: jit: don't rely on USE flag to detect support

Since ea68d36 we show whether JIT is enabled, based on the USE-flag
(USE_PCRE_JIT). This is too naive; libpcre may be built without JIT
support (which is the default).

Fix this by calling pcre_config(), which has the accurate information
we are looking for.

Example of a libpcre without JIT support after this patch:
> ./haproxy -vv | grep PCRE
>   OPTIONS = USE_STATIC_PCRE=1 USE_PCRE_JIT=1
> Built with PCRE version : 8.32 2012-11-30
> PCRE library supports JIT : no (libpcre build without JIT?)