Mujoco-py: Support latest homebrew gcc

[samuela]

Describe the bug
According to https://github.com/openai/mujoco-py/blob/master/mujoco_py/builder.py#L311 only gcc versions 6-8 are support on macOS. Now the latest version of gcc on homebrew is 9, which means that the install process is broken by default and the recommended fix:

brew install gcc

no longer works.

To Reproduce
Install homebrew gcc@9 and then attempt to install mujoco-py.

Expected behavior
mujoco-py to work with gcc@9.

Error Messages

Could not find GCC executable.\\n\\n'", '    RuntimeError: Could not find GCC executable.', '    ', '    HINT: On OS X, install GCC with `brew install gcc`. or `port install gcc`

Desktop (please complete the following information):

• OS: macOS 10.14.6
• Python Version 3.7.3
• Mujoco Version 200
• mujoco-py version 2.0.2.5

Environment

• output of: echo $LD_LIBRARY_PATH
• output of: echo $HOME
• output of: echo $USER

research 🔥 echo $LD_LIBRARY_PATH

research 🔥 echo $HOME
/Users/skainswo
research 🔥 echo $USER
skainswo

Additional context
n/a

[n-lavrenko]

The same error.
But downgrade of gcc to v.8 was helpful for me.
brew install gcc@8

[davidkiller96]

brew install gcc@8

thanks for the advice ~ . @n-lavrenko

[RyanRizzo96]

Issue still exists with gcc@9 and on macOS 10.15 I am unable to use gcc@8 too.