cl.h not found?

After a fresh clone, if I cd into OpenCL-ICD-Loader, and run

mkdir build
cd build
cmake ..

No problems. But if I then

make

it stops immediately with

Scanning dependencies of target OpenCL
[ 3%] Building C object CMakeFiles/OpenCL.dir/loader/icd.c.o
In file included from /khronos/github/OpenCL-ICD-Loader/loader/icd.c:19:
/khronos/github/OpenCL-ICD-Loader/loader/icd.h:36:10: fatal error: CL/cl.h: No such file or directory
36 | #include
| ^~~~~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/OpenCL.dir/build.make:82: CMakeFiles/OpenCL.dir/loader/icd.c.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:764: CMakeFiles/OpenCL.dir/all] Error 2
make: *** [Makefile:160: all] Error 2

Shouldn't I have to tell the build where the OpenCL headers are?

Reproduce the fresh-clone build with mkdir build, cmake .., and make, then inspect loader/icd.c, loader/icd.h, and the CMake configuration around the missing CL/cl.h include. Determine how the build is expected to locate the OpenCL headers; done means the documented build completes without the missing-header error.

Written by the indexing model from the issue text.