CMAKE build on Mac fails, 'string.h' file not found

[philburk]

On Mac, I used the instructions from the README.md:

cmake .
make

EXPECTED it to build.

ACTUAL result was this failure:

Scanning dependencies of target PForth_lib
[  2%] Building C object csrc/CMakeFiles/PForth_lib.dir/pf_cglue.c.o
In file included from /Users/phil/work/pforth/csrc/pf_cglue.c:22:
In file included from /Users/phil/work/pforth/csrc/pf_all.h:48:
/Users/phil/work/pforth/csrc/pf_inc1.h:26:14: fatal error: 'string.h' file not found
    #include <string.h>    /* Needed for strlen(), memcpy(), and memset(). */
             ^~~~~~~~~~
1 error generated.
make[2]: *** [csrc/CMakeFiles/PForth_lib.dir/pf_cglue.c.o] Error 1
make[1]: *** [csrc/CMakeFiles/PForth_lib.dir/all] Error 2