Build bullet libs for Pi 3

bullet_pi3

Notes for building Bullet-physics engine libraries for static linking into libBulletSim for Raspberry Pi 3.
libBulletSim is contained in git://opensimulator.org/git/opensim-libs

Still working on this:

1) sudo apt install libgl1-mesa-dev
2) download and unpack bullet3-2.86.1
3) cd bullet3-2.86.1
4) mkdir bullet-build
5) cd bullet-build
6) cmake .. -G "Unix Makefiles" -DBUILD_EXTRAS=ON -DBUILD_DEMOS=OFF -DBUILD_BULLET2_DEMOS=OFF -DBUILD_BULLET3=OFF -DBUILD_SHARED_LIBS=OFF -DINSTALL_LIBS=ON -DINSTALL_EXTRA_LIBS=ON -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_FLAGS="-fPIC"
7) make -j4
8) sudo make install

6) is the most critical one. A successful build does not guarantee proper execution of the final product.
   I'm not sure things are thread safe.