I have disabled building SDL with headless EGL, because
SDL_config_minimal.h doesn't work with EGL, and I don't know how to
implement a an SDL config that would work with EGL.
I have verified that this works in a separate project, and that it
compiles in this project.
```
$ ./build.sh -e release
$ find ./out/ -name "*EGL*"
./out/cmake-release/filament/backend/CMakeFiles/backend.dir/src/opengl/platforms/PlatformEGL.cpp.o
./out/cmake-release/filament/backend/CMakeFiles/backend.dir/src/opengl/platforms/PlatformEGLHeadless.cpp.o
./out/cmake-release/libs/bluegl/CMakeFiles/bluegl.dir/src/BlueGLLinuxEGL.cpp.o
```
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