ENH: Rehaul meson build script

[HaoZeke]

Part of #131. Essentially:

• DRY meson.build
• Add in options from legacy_setup.py
• Replace legacy_setup.py in CI

For reference, this is how openmp can be checked.

python -m pip install -v . -Csetup-args=-Duse_openmp=true
ldd /home/rgoswami/Git/Github/Quansight/freeThreading/scs-python/.ftci/lib/python3.13t/site-packages/scs/_scs_direct.cpython-313t-x86_64-linux-gnu.so
        linux-vdso.so.1 (0x00007f40db73f000)
        libblas.so.3 => /usr/lib/libblas.so.3 (0x00007f40db625000)
        liblapack.so.3 => /usr/lib/liblapack.so.3 (0x00007f40da600000)
        **libgomp.so.1 => /usr/lib/libgomp.so.1 (0x00007f40db5d1000)**
        libc.so.6 => /usr/lib/libc.so.6 (0x00007f40da410000)
        libgfortran.so.5 => /usr/lib/libgfortran.so.5 (0x00007f40da000000)
        libm.so.6 => /usr/lib/libm.so.6 (0x00007f40db4d7000)
        libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007f40db4aa000)
        /usr/lib64/ld-linux-x86-64.so.2 (0x00007f40db741000)

[HaoZeke]

The GPU bit is speculative (didn't have one to test).