Running 'make' on OS X gives error

[Woetjo]

When running make on the latest build of XCODE Version 10.2 (10E125) on OS X the following error messages appear:

Roels-MacBook-Pro:keyman64-1.6 Roel$ make
cc -std=gnu99 -Wall -Wno-unused -Wno-expansion-to-defined -O2 -DVERSION=1.6 -o keyman64 strings.c range.c usb.c intelhex.c keyman64.c -lusb-1.0
In file included from keyman64.c:23:
./keyman64.h:52:42: warning: implicit declaration of function 'Sleep' is invalid
      in C99 [-Wimplicit-function-declaration]
  unsigned int sleep(unsigned int sec) { Sleep(sec*1000); return 0; }
                                         ^
1 warning generated.
Undefined symbols for architecture x86_64:
  "_Sleep", referenced from:
      _sleep in keyman64-3f1019.o
      _expect in keyman64-3f1019.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [keyman64] Error 1