Revert "external/libgltf: Hack in missing -fPIC"

This reverts commit 5da85a21241f09a0b2d7796bc952806a1a863592,
bb9078e3dee48afc4c2bd8006a892924f8b38b39 "libgltf: use --with-pick [sic]" had
already added --with-pic.