Change default linux compiler to gcc. not clang

[arthursoprana]

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.

[volpatto]

I will work on it.