Check for includes in /usr/include/ffmpeg/libav*

[adaugherity]

Nowadays some distros (e.g. openSUSE/packman and Fedora/RPMFusion) are putting the ffmpeg includes under /usr/include/ffmpeg, e.g. /usr/include/ffmpeg/libavcodec/avcodec.h. Patch configure to find these.

Using pkg-config might be a better/more portable option, since ffmpeg-devel includes pkg-config files, but it's more difficult. I got a pkg-config version working using PKG_CHECK_MODULES and PHP_ADD_INCLINE, but only by running aclocal; autoconf after phpize and before configure, as for some reason phpize isn't loading /usr/share/aclocal/pkg.m4. Even that is way further down the autotools rabbit-hole than I ever wished to venture, so I decided to submit the simpler patch instead.