Set GCC's -fno-common flag for C compilation if supported

Dylan Baker requested to merge dbaker/mesa:gcc-fno-common into master

GCC < 10 sets -fcommon by default, GCC >= 10 sets -fno-common by default. In order to avoid "works for me" problems across the changed default lets pick one and always use it. -fno-common makes more sense as it reduces code size and can increase performance, and works the way many other operating systems do. This is only needed for C as the option doesn't affect C++.

CC: 20.0

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