Trying to build the Loader causes error

I've been trying to build it with instructions from readme, but whenever I try to build in step 5, I get this error:

```
D:\OpenCL-ICD-Loader\build> make

make[1]: Entering directory '/cygdrive/d/OpenCL-ICD-Loader/build'
make[2]: Entering directory '/cygdrive/d/OpenCL-ICD-Loader/build'
Scanning dependencies of target OpenCL
make[2]: Leaving directory '/cygdrive/d/OpenCL-ICD-Loader/build'
make[2]: Entering directory '/cygdrive/d/OpenCL-ICD-Loader/build'
[ 4%] Building C object CMakeFiles/OpenCL.dir/loader/icd.c.o
/cygdrive/d/OpenCL-ICD-Loader/loader/icd.c: In function 'loader_get_next_path':
/cygdrive/d/OpenCL-ICD-Loader/loader/icd.c:199:25: error: 'PATH_SEPARATOR' undeclared (first use in this function)
199 | next = strchr(path, PATH_SEPARATOR);
| ^~~~~~~~~~~~~~
/cygdrive/d/OpenCL-ICD-Loader/loader/icd.c:199:25: note: each undeclared identifier is reported only once for each function it appears in
make[2]: *** [CMakeFiles/OpenCL.dir/build.make:63: CMakeFiles/OpenCL.dir/loader/icd.c.o] Error 1
make[2]: Leaving directory '/cygdrive/d/OpenCL-ICD-Loader/build'
make[1]: *** [CMakeFiles/Makefile2:297: CMakeFiles/OpenCL.dir/all] Error 2
make[1]: Leaving directory '/cygdrive/d/OpenCL-ICD-Loader/build'
make: *** [Makefile:141: all] Error 2
```

Start with README step 5 and the failing build output, then inspect loader/icd.c around line 199 where loader_get_next_path reports PATH_SEPARATOR as undeclared. Confirm the fix by rerunning make from the build directory and verifying that the OpenCL target builds successfully.

Written by the indexing model from the issue text.