Caching problem with building on CPU MacOSX Mojave ?

Since I have update to Mojave, I get building failures on CPU device.

If I run
```import numpy as np
import pyopencl as cl

a_np = np.random.rand(50000).astype(np.float32)
b_np = np.random.rand(50000).astype(np.float32)

ctx = cl.create_some_context()
queue = cl.CommandQueue(ctx)

mf = cl.mem_flags
a_g = cl.Buffer(ctx, mf.READ_ONLY | mf.COPY_HOST_PTR, hostbuf=a_np)
b_g = cl.Buffer(ctx, mf.READ_ONLY | mf.COPY_HOST_PTR, hostbuf=b_np)

prg = cl.Program(ctx, """
__kernel void sum(
__global const float *a_g, __global const float *b_g, __global float *res_g)
{
int gid = get_global_id(0);
res_g[gid] = a_g[gid] + b_g[gid];
}
""")

prg=prg.build()

res_g = cl.Buffer(ctx, mf.WRITE_ONLY, a_np.nbytes)
prg.sum(queue, a_np.shape, None, a_g, b_g, res_g)

res_np = np.empty_like(a_np)
cl.enqueue_copy(queue, res_np, res_g)

# Check on CPU with Numpy:
print(res_np - (a_np + b_np))
print(np.linalg.norm(res_np - (a_np + b_np)))
```

it works only once. Every attempt to build the same Program again will raise

```
RuntimeError: clBuildProgram failed: BUILD_PROGRAM_FAILURE - clBuildProgram failed: BUILD_PROGRAM_FAILURE - clBuildProgram failed: BUILD_PROGRAM_FAILURE

Build on :
```

If I change the kernel source, _e.g._ inserting a blank line or a comment, the build will succeed but only once. I suspect it has something to do with the caching mechanism somewhere.

Apparently related to #252, because when I run for the first time the code there, it succeeded, but now I always get the build failure.

EDIT
If some build option is added _e.g._

```
prg=prg.build(options=['-cl-opt-disable'])
```

the build will succeed once, and then will fail again. If the option is changed, it will succeed once then fail, etc.