make error on macOS use gcc 7
```
$ cmake ..  -DCMAKE_C_COMPILER=gcc-7 -DCMAKE_CXX_COMPILER=g++-7
-- The C compiler identification is GNU 7.2.0
-- The CXX compiler identification is GNU 7.2.0
-- Checking whether C compiler has -isysroot
-- Checking whether C compiler has -isysroot - yes
-- Checking whether C compiler supports OSX deployment target flag
-- Checking whether C compiler supports OSX deployment target flag - yes
-- Check for working C compiler: /usr/local/bin/gcc-7
-- Check for working C compiler: /usr/local/bin/gcc-7 -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Checking whether CXX compiler has -isysroot
-- Checking whether CXX compiler has -isysroot - yes
-- Checking whether CXX compiler supports OSX deployment target flag
-- Checking whether CXX compiler supports OSX deployment target flag - yes
-- Check for working CXX compiler: /usr/local/bin/g++-7
-- Check for working CXX compiler: /usr/local/bin/g++-7 -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found UV: /usr/local/lib/libuv.dylib  
-- Looking for syslog.h
-- Looking for syslog.h - not found
-- Found mhd: /usr/local/include  
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/xxx/git/xmrig/build
```

```
$ make
Scanning dependencies of target cpuid
[  2%] Building C object src/3rdparty/libcpuid/CMakeFiles/cpuid.dir/cpuid_main.c.o
[  4%] Building C object src/3rdparty/libcpuid/CMakeFiles/cpuid.dir/asm-bits.c.o
[  6%] Building C object src/3rdparty/libcpuid/CMakeFiles/cpuid.dir/recog_amd.c.o
[  8%] Building C object src/3rdparty/libcpuid/CMakeFiles/cpuid.dir/recog_intel.c.o
[ 10%] Building C object src/3rdparty/libcpuid/CMakeFiles/cpuid.dir/libcpuid_util.c.o
[ 13%] Linking C static library libcpuid.a
[ 13%] Built target cpuid
Scanning dependencies of target xmrig
[ 15%] Building CXX object CMakeFiles/xmrig.dir/src/api/Api.cpp.o
[ 17%] Building CXX object CMakeFiles/xmrig.dir/src/api/ApiState.cpp.o
[ 19%] Building CXX object CMakeFiles/xmrig.dir/src/api/NetworkState.cpp.o
[ 21%] Building CXX object CMakeFiles/xmrig.dir/src/App.cpp.o
[ 23%] Building CXX object CMakeFiles/xmrig.dir/src/Console.cpp.o
[ 26%] Building CXX object CMakeFiles/xmrig.dir/src/log/ConsoleLog.cpp.o
[ 28%] Building CXX object CMakeFiles/xmrig.dir/src/log/FileLog.cpp.o
[ 30%] Building CXX object CMakeFiles/xmrig.dir/src/log/Log.cpp.o
[ 32%] Building CXX object CMakeFiles/xmrig.dir/src/Mem.cpp.o
[ 34%] Building CXX object CMakeFiles/xmrig.dir/src/net/Client.cpp.o
[ 36%] Building CXX object CMakeFiles/xmrig.dir/src/net/Job.cpp.o
[ 39%] Building CXX object CMakeFiles/xmrig.dir/src/net/Network.cpp.o
[ 41%] Building CXX object CMakeFiles/xmrig.dir/src/net/strategies/DonateStrategy.cpp.o
[ 43%] Building CXX object CMakeFiles/xmrig.dir/src/net/strategies/FailoverStrategy.cpp.o
[ 45%] Building CXX object CMakeFiles/xmrig.dir/src/net/strategies/SinglePoolStrategy.cpp.o
[ 47%] Building CXX object CMakeFiles/xmrig.dir/src/net/SubmitResult.cpp.o
[ 50%] Building CXX object CMakeFiles/xmrig.dir/src/net/Url.cpp.o
[ 52%] Building CXX object CMakeFiles/xmrig.dir/src/Options.cpp.o
[ 54%] Building CXX object CMakeFiles/xmrig.dir/src/Platform.cpp.o
[ 56%] Building CXX object CMakeFiles/xmrig.dir/src/Summary.cpp.o
[ 58%] Building CXX object CMakeFiles/xmrig.dir/src/workers/DoubleWorker.cpp.o
[ 60%] Building CXX object CMakeFiles/xmrig.dir/src/workers/Handle.cpp.o
[ 63%] Building CXX object CMakeFiles/xmrig.dir/src/workers/Hashrate.cpp.o
[ 65%] Building CXX object CMakeFiles/xmrig.dir/src/workers/SingleWorker.cpp.o
[ 67%] Building CXX object CMakeFiles/xmrig.dir/src/workers/Worker.cpp.o
[ 69%] Building CXX object CMakeFiles/xmrig.dir/src/workers/Workers.cpp.o
[ 71%] Building CXX object CMakeFiles/xmrig.dir/src/xmrig.cpp.o
[ 73%] Building CXX object CMakeFiles/xmrig.dir/src/App_unix.cpp.o
[ 76%] Building CXX object CMakeFiles/xmrig.dir/src/Cpu_mac.cpp.o
[ 78%] Building CXX object CMakeFiles/xmrig.dir/src/Mem_unix.cpp.o
[ 80%] Building CXX object CMakeFiles/xmrig.dir/src/Platform_mac.cpp.o
/Users/xxx/git/xmrig/src/Platform_mac.cpp: In function 'char* createUserAgent()':
/Users/xxx/git/xmrig/src/Platform_mac.cpp:48:129: error: '__clang_major__' was not declared in this scope
     snprintf(buf, max, "%s/%s (Macintosh; Intel Mac OS X) libuv/%s clang/%d.%d.%d", APP_NAME, APP_VERSION, uv_version_string(), __clang_major__, __clang_minor__, __clang_patchlevel__);

/Users/xxx/git/xmrig/src/Platform_mac.cpp:48:146: error: '__clang_minor__' was not declared in this scope
     snprintf(buf, max, "%s/%s (Macintosh; Intel Mac OS X) libuv/%s clang/%d.%d.%d", APP_NAME, APP_VERSION, uv_version_string(), __clang_major__, __clang_minor__, __clang_patchlevel__);
  
/Users/xxx/git/xmrig/src/Platform_mac.cpp:48:163: error: '__clang_patchlevel__' was not declared in this scope
     snprintf(buf, max, "%s/%s (Macintosh; Intel Mac OS X) libuv/%s clang/%d.%d.%d", APP_NAME, APP_VERSION, uv_version_string(), __clang_major__, __clang_minor__, __clang_patchlevel__);

make[2]: *** [CMakeFiles/xmrig.dir/src/Platform_mac.cpp.o] Error 1
make[1]: *** [CMakeFiles/xmrig.dir/all] Error 2
make: *** [all] Error 2
```