Do not leak compile options outside project

[vocasle]

This change reduces the scope of compile options from other CMake targets and applies them only to targets defined in root CMakeLists.txt.

add_compile_options sets compile options to all CMake targets.
target_compile_options allows to narrow the scope to targets defined in root CMakeLists.txt.

[zeux]

add_compile_options sets compile options to all CMake targets

I was under the impression that this is already scoped to targets defined in the folder (/subdirectories) of the current CMakeLists file? Iow it should not propagate to targets defined outside of meshoptimizer source.

[vocasle]

My apologies. I was looking for the root cause of-Wmissing-field-initializer propagating to error and stumbled upon -Wno-missing-field-initializer, but failed to notice Wno.

This is an excerpt from compile_commands.json of executable that links with meshoptimizer, the executable does not contain any compile flags from meshoptimizer :

{
  "directory": "/Users/user/Source/meshopt-leaking-compile-options/build",
  "command": "/usr/bin/c++  -I/Users/user/Source/meshopt-leaking-compile-options/meshoptimizer/src  -arch arm64 -o CMakeFiles/test.dir/test.cpp.o -c /Users/user/Source/meshopt-leaking-compile-options/test.cpp",
  "file": "/Users/user/Source/meshopt-leaking-compile-options/test.cpp",
  "output": "CMakeFiles/test.dir/test.cpp.o"
},
{
  "directory": "/Users/user/Source/meshopt-leaking-compile-options/build/meshoptimizer",
  "command": "/usr/bin/c++    -arch arm64 -Wall -Wextra -Wshadow -Wno-missing-field-initializers -o CMakeFiles/meshoptimizer.dir/src/allocator.cpp.o -c /Users/user/Source/meshopt-leaking-compile-options/meshoptimizer/src/allocator.cpp",
  "file": "/Users/user/Source/meshopt-leaking-compile-options/meshoptimizer/src/allocator.cpp",
  "output": "meshoptimizer/CMakeFiles/meshoptimizer.dir/src/allocator.cpp.o"
}

Sorry for the hustle.
Closing PR.