Add Compiler flag -fPIC

This fixes an issue on `x86_64` machines where linking to `libaxe.a` would throw
```
relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC
```

CMakeLists.txt

@@ -15,7 +15,7 @@ if(COMPILER_IS_GCC_COMPATIBLE)
   check_cxx_compiler_flag("-std=c++11" SUPPORTS_CXX11)
   if(SUPPORTS_CXX11)
     message(STATUS "-std=c++11 is supported.")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fPIC -std=c++11")
   else()
     check_cxx_compiler_flag("-std=c++0x" SUPPORTS_CXX0X)
     if(SUPPORTS_CXX0X)
@@ -24,8 +24,8 @@ if(COMPILER_IS_GCC_COMPATIBLE)
     else()
       message(FATAL_ERROR "C++11 is unsupported.")
     endif()
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -W -Wno-unused-parameter")
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -W -Wno-unused-parameter")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -W -Wno-unused-parameter -fPIC")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -W -Wno-unused-parameter -fPIC")
   endif()
 elseif(MSVC)
 # Disable the following warnings: