Respect STDEXEC_ENABLE_LIBDISPATCH option

[fallintoplace]

Summary

Make the Darwin libdispatch compile definition follow STDEXEC_ENABLE_LIBDISPATCH instead of defining it unconditionally whenever CMake runs on Darwin. This lets users explicitly configure with -DSTDEXEC_ENABLE_LIBDISPATCH=OFF even when dispatch/dispatch.h is present.

Testing

• cmake -S . -B build-libdispatch-off -G Ninja -DSTDEXEC_ENABLE_LIBDISPATCH=OFF
• confirmed build-libdispatch-off/compile_commands.json does not contain STDEXEC_ENABLE_LIBDISPATCH
• cmake --build build-libdispatch-off --target test.exec -j 8
• ./build-libdispatch-off/test/exec/test.exec
• cmake -S . -B build -G Ninja
• confirmed build/compile_commands.json contains STDEXEC_ENABLE_LIBDISPATCH with the default Darwin option
• cmake --build build --target test.exec -j 8
• ./build/test/exec/test.exec "*libdispatch*"
• ./build/test/exec/test.exec

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[ericniebler]

/ok to test c5116ea