#2249 Add gentoo's gcc-config to pick proper gcc for CUDA compile

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Use the (export $(gcc-config -E <index>) for the x86_64-pc-linux-gnu-14 entry before setup.py runs, so cmake and nvcc see gcc 14 instead of the 15 that dies in that build; STRATA_GCC_CONFIG picks another entry, empty turns the step off, and without gcc-config every other Linux is untouched)