Toolchain CMake files for clang currently pass -ffreestanding option
to prevent clang from picking the wrong copy of stdint.h. This is a
temporary hack until we fix clang distributions to not include the
GCC version of stdint.h. This hack, however, results in setting
__STDC_HOSTED__=0, which is being checked by cbor header files,
making some required functions unavailable as a result.
Undefine __STDC_HOSTED__ as a workaround. This flag is only passed
when compiling cbor itself.