graphics/blender: fix build on GCC architectures

[pkubaj]

Summary

Error:
/wrkdirs/usr/ports/graphics/blender/work/blender-2.79b/source/blender/blenlib/intern/math_base.c: In function 'double_round':
/wrkdirs/usr/ports/graphics/blender/work/blender-2.79b/source/blender/blenlib/intern/math_base.c:59:17: error: conversion from 'double' to 'float' may change value [-Werror=float-conversion]

if (!isfinite(y))

Fix by adding -Wno-float-conversion.

Also add USES=gl gnome sdl.

PR: 239331

[tcberner]