Okay, so, just to update to bring this to some close.
To solve this error of it not finding vpx >= 1.3.0, despite my version of vpx being 1.6.1, I first made sure that pkg-config was working correctly. This required downloading and copying the correct DLLs on the Windows side, which can be gotten, for example, from
codeblocks-17.12mingw-nosetup.zip in mingw/bin
Once pkg-config can be run without complaining about needing a DLL to run, I ran
pkg-config --exists --print-errors vpx >= 1.3.0
This gives a hint,
Package vpx was not found in the pkg-config search path.
Perhaps you should add the directory containing `vpx.pc'
to the PKG_CONFIG_PATH environment variable
is in my libvpx-master directory, so I added that path to PKG_CONFIG_PATH.
I ran the following commands:
./configure --enable-libmp3lame --enable-libvpx --extra-ldflags="-L/c/dev/libs/lame-3.100/libmp3lame/.libs" --extra-cflags="-I/c/dev/libs/lame-3.100/include" --extra-ldflags="-L/c/dev/libs/libvpx-master" --extra-cflags="-I/c/dev/libs/libvpx-master"
And configuration seems to have completed successfully! Hopefully the rest of the installation will work as well.
Maybe this will help someone that might google this in the future, haha.
make and make install worked as well.
I was able to compile the sample project located at https://libav.org/documentation/doxygen/master/encode__video_8c_source.html
with the following command line.
g++.exe -LC:\dev\libs\libav-12.2\libavcodec -LC:\dev\libs\libav-12.2\libavformat -LC:\dev\libs\libav-12.2\libavutil -LC:\dev\libs\lame-3.100\libmp3lame\.libs -LC:\dev\libs\libvpx-master -LC:\dev\libs\libav-12.2\libavresample -o bin\Debug\libav_test.exe obj\Debug\main.o -lavcodec -lavformat -lavutil -lmp3lame -lvpx -lavresample
The file it makes is 50 MB, although this is also without optimizations... I guess that's what static linking a bunch of things inevitably does.