If we enable a component but a dependant library is disabled, then the enabled
component gets silently disabled. Warning about disabled explicitly enabled components
allows configure to show the missing dependencies and if --fatal-warnings is
used it can also fail if the user wants it so.

For example if libdav1d is not availble ./configure --enable-decoder=libdav1d
succeeds but the libdav1d decoder is not be enabled. After the patch configure
will warn about this:

WARNING: Disabled libdav1d_decoder because not all dependencies are satisfied: libdav1d
Signed-off-by: Marton Balint <cus@passwd.hu>