`armhf` builds fail with vendored libraries

[Semphriss]

When compiling SDL_image with -DSDLIMAGE_VENDORED=ON, some of the libraries (specifically aom) fail to build with various errors like error: inlining failed in call to always_inline ‘vsetq_lane_u32’: target specific option mismatch.

I am not familiar enough with that error message to know whether this is a bug with the library itself, the command line arguments given to the compiler, or if the library just doesn't support armhf and SDL mistakenly attempts to compile it anyways, so I open the issue here for guidance on how to proceed.

[madebr]

I think you should look at how package managers handle this, and if that does not work out, ask upstream.
(Let us know how you fixed this issue)

[Semphriss]

I found that Debian passes -DAOM_NEON_INTRIN_FLAG=-mfpu=neon, I'll share my findings if I can manage to compile it.

[Semphriss]

Simply passing the argument to CMake when compiling SDL_Image works; I added a set(AOM_NEON_INTRIN_FLAG -mfpu=neon) before add_subdirectory(external/aom EXCLUDE_FROM_ALL) and it works as well.

I'm not sure if that option is the same across compilers so I'm a bit hesitant to open a PR right away; what would be the best way to proceed?