Older cards on Mac stopped working.
Hi,

I cannot run 2.8.x on older cards on Mac. Previous version was OK. Macs with RPro 575 works well.

```
MAC:xmrig-amd root# ./xmrig-amd
 * ABOUT        XMRig-AMD/2.8.3 clang/8.0.0
 * LIBS         libuv/1.18.0 OpenCL/1.2 OpenSSL/1.0.2p microhttpd/0.9.59
 * CPU          Intel(R) Xeon(R) CPU           W3530  @ 2.80GHz x64 -AES
 * ALGO         cryptonight, donate=5%
 * POOL #1      192.168.1.100:7777 variant auto
 * COMMANDS     hashrate, pause, resume
[2018-10-19 16:28:58] compiling code and initializing GPUs. This will take a while...
[2018-10-19 16:28:58] #0, GPU #0 ATI Radeon HD 5770, intensity: 232 (8/256), unroll: 8, cu: 10
[2018-10-19 16:28:58] GPU #0 compiling...
[2018-10-19 16:29:12] Error CL_BUILD_PROGRAM_FAILURE when calling clBuildProgram.
Build log:
Error returned by cvms_element_build_from_source
[2018-10-19 16:29:12] Error CL_INVALID_PROGRAM_EXECUTABLE when calling clCreateKernel for kernel cn0.
[2018-10-19 16:29:12] Failed to start threads.
```
Anyone?