gmp.h not found on OS X

[girving]

If I try to build geode on my Mac OS X Sierra laptop, cmake . gives

-- GMP found in /usr/local/lib/libgmp.dylib
-- Python found. Building python bindings
-- Numpy found in /usr/local/lib/python2.7/site-packages/numpy/core/include
-- Building array module.
-- Building python module.
-- Building force module.
-- Building utility module.
-- Building math module.
-- Building geometry module.
-- Building random module.
-- Building structure module.
-- Building solver module.
-- Building image module.
-- Building svg_to_bezier module.
-- Building mesh module.
-- Building value module.
-- Building vector module.
-- Building exact module.
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/irving/geode

Unfortunately, even though gmp.h was correctly located in /usr/local/include, building gives

(master) emmy:geode% cns -j 1
[  0%] Building CXX object geode/exact/CMakeFiles/exact.dir/circle_csg.cpp.o
In file included from /Users/irving/geode/geode/exact/circle_csg.cpp:11:
/Users/irving/geode/geode/exact/Exact.h:15:10: fatal error: 'gmp.h' file not found
#include <gmp.h>
         ^
1 error generated.
make[2]: *** [geode/exact/CMakeFiles/exact.dir/circle_csg.cpp.o] Error 1
make[1]: *** [geode/exact/CMakeFiles/exact.dir/all] Error 2
make: *** [all] Error 2

The geode cmake set up is new to me, so I'm not sure how one fixes this.