Description of problem:
Pocl needs to be able to compile OpenCL kernels in order to be used. 
it depends on the 'compiler-rt' package (which is part of clang) in order to do this. 

It requires the file: /usr/lib64/clang/6.0.0/include/opencl-c.h
which is provided by the main clang package to be available in order to compile many OpenCL kernels

Else the build fails:  
error: <built-in>:2:10: '/usr/lib64/clang/6.0.0/include/opencl-c.h' file not found


Version-Release number of selected component (if applicable):
pocl-1.1-2

How reproducible:
Always

Steps to Reproduce:
1. Install pocl, compiler-rt, pyopencl, and other dependencies
2. make sure clang  is NOT installed (rpm -e clang)
3. Run the following Python testcase
import pyopencl as cl
ctx = cl.create_some_context()
prg = cl.Program(ctx, """
__kernel void testcase(
    __global const float *a_g, __global float *res_g)
{
  int gid = get_global_id(0);
  res_g[gid] = a_g[gid]*M_PI;
}
""").build()


Actual results:
error: <built-in>:2:10: '/usr/lib64/clang/6.0.0/include/opencl-c.h' file not found

Expected results:
Successful compile of OpenCL kernel


Additional info:
Possible fix: 
  * pocl could depend on clang as well as compiler-rt