sync-build.sh: switch to lld, remove unused stuff

Remove ENABLE_xxx options that are enabled by default, check
CMakeOptions.txt instead if you want to see available values.
Skip rebuilding if only tests change. Remove -fsanitize options since
these are set via CMake options.

Use lld instead of gold since it is almost twice as fast as gold:

    lld     4.8s
    gold    9.2s
    bfd    10.8s

Tested (n=3) with lld 7.0.1-1 and binutils 2.31.1-4 on Arch Linux, Linux
4.19.12-arch1-1-ARCH, i7-6700HQ, wireshark v2.9.1rc0-363-g178d81f804.
Commands:

    time cmake -GNinja /tmp/wireshark -DCMAKE_BUILD_TYPE=Debug \
        -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \
        -DCMAKE_C_FLAGS="-fdiagnostics-color -fuse-ld=lld" \
        -DCMAKE_CXX_FLAGS="-fdiagnostics-color -fuse-ld=lld" \
        -DDISABLE_WERROR=1 -DENABLE_ASAN=1 -DENABLE_UBSAN=1
    time ninja && rm -rf run
    time ninja      # measured this with various -fuse-ld= values.

Total build time is about 21s for cmake, 3m18s for the initial ninja.
Ignoring the noise, a full build somehow takes about the same time as
with gold, but incremental builds with lld are consistently faster.

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