Change default linux compiler to gcc. not clang

### Description

The current cmake configuration uses 

```
if(NOT WIN32)
  set(CMAKE_C_COMPILER    clang)
  set(CMAKE_CXX_COMPILER  clang++)
  set(CMAKE_C_FLAGS       "-Wall -std=c99")
  set(CMAKE_C_FLAGS_DEBUG "-g")
endif(NOT WIN32)
```

I think it should be `gcc` instead of `clang`, if possible.
