util: Define strchrnul on macOS.
strchrnul is not available on macOS.
pipe_loader.c:141:14: error: implicit declaration of function 'strchrnul' is invalid in C99 [-Werror,-Wimplicit-function-declaration] next = strchrnul(library_paths, ':'); ^
Signed-off-by: Vinson Lee vlee@freedesktop.org