update mp4 output to newer gpac versions
I updated the configure and mp4 output code to work with newer version of gpac, namely 0.8.0 and up (including the newest 1.0.0 branch).
The biggest relevant change here is that
gf_isom_set_pixel_aspect_ratio() now takes a sixth argument.
Tested it on ubuntu 18.04, macos (although it needs some extra ld flags there) and windows+mingw.