doc: Mention that DragonFly BSD requires __BSD_VISIBLE set

Setting this should fix the FATE build failure on DragonFly BSD,
since inet_aton only is visible if __BSD_VISIBLE is set.

Alternatively, a line defining __BSD_VISIBLE=1 could be
added at the top of os_support.c. For FreeBSD, similar lines
are required in libavdevice/bktr.c and libavdevice/oss_audio.c, too.

Signed-off-by: Martin Storsjö <martin@martin.st>

doc/general.texi

@@ -784,9 +784,9 @@ to configure.
 BSD make will not build Libav, you need to install and use GNU Make
 (@file{gmake}).
 
-@subsubsection FreeBSD
+@subsubsection FreeBSD, DragonFly BSD
 
-FreeBSD will not compile out-of-the-box due to broken system headers.
+These systems will not compile out-of-the-box due to broken system headers.
 Passing @code{--extra-cflags=-D__BSD_VISIBLE} to configure will work
 around the problem. This may have unexpected sideeffects, so use it at
 your own risk. If you care about FreeBSD, please make an attempt at