haiku: fix Mesa build
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The hgl.c file is a read-only file versus read-write. Ref: src/gallium/state_trackers/hgl/hgl.c
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I've included the Haiku-specific patches I used to get a successful build of Mesa 19.1.7 on Haiku using the meson/ninja build procedure. Shows "[764/764] linking target ... libswpipe.so" at build completion.
Reported-by: Ken Mays kmays2000@gmail.com Tested-by: Ken Mays kmays2000@gmail.com CC: 19.1 mesa-stable@lists.freedesktop.org