Your compiler output does look a little off. There's no explicit include of/usr/local/includeon my end: gcc -c -fno-strict-overflow -Wsign-compare -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -g -Og -Wall -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers ...
warning: mujoco_py/generated/wrappers.pxi:4:0: Dotted filenames ('../pxd/mujoco.pxd') are deprecated. Please use the normal Python package directory layout. running build_ext building 'mujoco_py.cymj' extension /usr/local/bin/gcc-9 -Wno-unused-result -Wsign-compare -Wunreachable-code -fn...