workaround clang 14 __has_declspec_attribute restriction

[liam-x]

fix #1710

clang 14 impose a new restriction on __has_declspec_attribute, the argument can not be empty. https://github.com/llvm/llvm-project/issues/53269

cppresetsdk cpprest_compat.h translate dllimport to empty which cause compilation failure on clang 14.

A workaround by removing define dllimport and add no-unknown-attributes flag to suppress the compilation warning from clang on macOS and Linux.