docs: drop most autoconf references

There's still a few in here, but those docs are already so out of date
that it probably makes more sense to delete them. Such as the GLES
docs which still claim we only support 1.1 and 2.0, with no mention of
3.x at all.

v2: - Add docs for testing back end (Eric Engestrom)
    - Drop more autootols references
    - meson is now required not recommended
    - Add $PWD

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Matt Turner <mattst88@gmail.com>

docs/osmesa.html

@@ -51,8 +51,8 @@ There are several examples of OSMesa in the mesa/demos repository.
 Configure and build Mesa with something like:
 
 <pre>
-configure --enable-osmesa --disable-driglx-direct --disable-dri --with-gallium-drivers=swrast
-make
+meson builddir -Dosmesa=gallium -Dgallium-drivers=swrast -Ddri-drivers= -Dvulkan-drivers= -Dprefix=$PWD/builddir/install
+ninja -C builddir install
 </pre>
 
 <p>
@@ -63,13 +63,12 @@ Make sure you have LLVM installed first if you want to use the llvmpipe driver.
 When the build is complete you should find:
 </p>
 <pre>
-lib/libOSMesa.so  (swrast-based OSMesa)
-lib/gallium/libOSMsea.so  (gallium-based OSMesa)
+$PWD/builddir/install/lib/libOSMesa.so  (swrast-based OSMesa)
+$PWD/builddir/install/lib/gallium/libOSMsea.so  (gallium-based OSMesa)
 </pre>
 
 <p>
-Set your LD_LIBRARY_PATH to point to one directory or the other to select
-the library you want to use.
+Set your LD_LIBRARY_PATH to point to $PWD/builddir/install to use the libraries
 </p>
 
 <p>