Add FreeBSD subsection with compilation instructions.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22686 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

doc/general.texi

@@ -720,6 +720,14 @@ performance on systems without hardware floating point support).
 BSD make will not build FFmpeg, you need to install and use GNU Make
 (@file{gmake}).
 
+@subsection FreeBSD
+
+FreeBSD 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
+getting the system headers fixed.
+
 @section Windows
 
 To get help and instructions for building FFmpeg under Windows, check out