helper/compiler: fix build on MacOS
On MacOS, clang defines [1]:
#define __nonnull _Nonnull
that creates incompatibility with GCC and with the macro __nonnull
defined in some libc.
Detect clang on MacOS and undefine __nonnull.
Change-Id: I64fcf51b102ea91c196e657debd8c267943a2b08
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Links: [1] https://github.com/llvm/llvm-project/blob/llvmorg-16.0.0/clang/lib/Frontend/InitPreprocessor.cpp#L1226
Reviewed-on: https://review.openocd.org/c/openocd/+/7544
Tested-by: jenkins