Split CMakeLists.txt into cmake/Plugin.cmake (production formats and
MinGW link flags) and cmake/Harness.cmake (console test harness with
CTest registration). Add SERUMALT_BUILD_PLUGIN and SERUMALT_BUILD_TESTS
options so production and harness builds are mutually exclusive and
each omits the other's targets.
Add build_linux.sh, build_macos.sh, and build_harness.sh with host
validation, env-overridable build dirs, and explicit option flags.
Rewrite build_windows.sh to use the checked-in mingw64-toolchain.cmake
instead of regenerating it, validate all required tools, and stage
with cmake -E copy_directory.
Broaden .gitignore to cover all build_*/ directories.
biggy
f10409ad1e