INSTALL.md: Add information about bpf_dump.c

OpenBSD libpcap does not implement bpf_dump().

That's why we need to keep the code and the checks in CMakeLists.txt and
configure.ac even if the official libpcap implement bpf_dump() since
version 0.6.

See the test in PR #1094.

Author: fxlb

INSTALL.md

@@ -82,7 +82,7 @@ atime.awk	- TCP ack awk script
 atm.h		- ATM traffic type definitions
 autogen.sh	- build configure and config.h.in (run this first)
 bpf_dump.c	- BPF program printing routines, in case libpcap doesn't
-		  have them
+		  have them. A known example is OpenBSD libpcap.
 chdlc.h		- Cisco HDLC definitions
 cpack.c		- functions to extract packed data
 cpack.h		- declarations of functions to extract packed data