devel/godot2: disable internal libs
Remove use of python2 for scons, builds fine with py3 scons
Disable use of internal png, freetype and zlib.
This matches ports for godot v3 and fixes build on powerpc64le.
PR: 254867
Approved by: maintainer
MFH: 2021Q2
(cherry picked from commit 56c603914f70b67f04164958ab80f7a0ebd2c0e0)