CMakeLists: Fixup boost

* boost is part of the public interface.
* Consider boost a system library so warnings from boost do not cause a build failure.
* If the parent project defines boost, use that.

src/CMakeLists.txt

@@ -112,8 +112,9 @@ target_include_directories(dynarmic
                            PRIVATE .)
 target_compile_options(dynarmic PRIVATE ${DYNARMIC_CXX_FLAGS})
 target_link_libraries(dynarmic
-    PRIVATE
+    PUBLIC
         boost
+    PRIVATE
         fmt::fmt
         xbyak
         $<$<BOOL:DYNARMIC_USE_LLVM>:${llvm_libs}>