Check for pcre_dfa_exec (RHEL4 libpcre doesn't have it)

[rainer]

On RHEL4 samhain build with --enable-logfile-monitor option fails:

sh_log_correlate.o(.text+0x47c): In function `sh_keep_match':
: undefined reference to `pcre_dfa_exec'
collect2: ld returned 1 exit status
make: *** [samhain] Error 1

(reported by Shinoj)

[rainer]

Fixed in changeset [285].