build: Report an error message when a pc file is not found

Ease tracking path problems.
mem: Document the av_realloc family of functions properly

realloc() does not accept pointers from memalign().
doc: Describe TB option of setpts filter

Signed-off-by: Diego Biurrun <diego@biurrun.de>
mpegvideo: Avoid 32-bit wrapping of linesize multiplications

This makes sure that linesize * start_y doesn't overflow, so that
emulated_edge_mc can get back the original value if needed.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
matroskaenc: Allow chapters to be written in trailer

This allows creation of frame accurate chapter marks from sources like
DVD and BD where the precise chapter location is not known until the
chapter mark has been reached during reading.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
mem: Improve documentation wording and spelling
mem: Do not check unsigned values for negative size
mpeg12enc: drop forward declarations

Signed-off-by: Diego Biurrun <diego@biurrun.de>