Clarify details about requirements in BUILDING.txt

BUILDING.txt

@@ -11,6 +11,8 @@ Build Requirements (All Systems)
 
 -- gcc
 
+Development kits for the following packages:
+
 -- zlib
 
 -- pixman
@@ -46,7 +48,9 @@ Build Requirements (Unix)
    * All build requirements Xorg imposes (see its documentation)
    * patch
 
+-- Optional ffmpeg development kit support (libav)
+   * You might have to enable additional repositories for this. E.g.,
+     on RHEL, EPEL and RPMFusion (free + nonfree) need to be enabled.
 
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 Build Requirements (Windows)