strange, it was working with those TWO lines in Slackbuild:

EXTRA_LIBS=" -lOpenCL -lSvtAv1Enc -lvpl -ldav1d -lxvidcore -lass -lbluray -lsnappy -lzimg -lplacebo -lshaderc_shared" \                                                                               FFMPEG_EXTRA_CFG=" --enable-libvpl --disable-doc --enable-opencl --enable-libsvtav1 --enable-frei0r --enable-libdav1d --enable-libzimg --enable-libxvid --enable-libass --enable-libbluray --enable-libsnappy --enable-libshaderc --enable-libplacebo --enable-vulkan --disable-debug --extra-cflags=-I/usr/local/include/vpl --extra-cflags=-I/usr/include/svt-av1" \

be sure to export both?

You can't use something.pc in -L statement directly, it sets directory where linker looks for libraries.

pkg-config is basically program (from system -  I saw it on Linux distros and NetBSD's pkgsrc) that checks those *.pc files and helps to construct include and libs lines. 

-I is also sets include search *directory* and not a file, as far as I understand.

But yeah, for example cmake sometimes mis-set those, so I tend to try and fix this on my slightly non-standart system. Or just give up and symlink few things into /usr