Android: define required __STDC* macros as cflags

Android: define required __STDC* macros as cflags

Necessary to fix the following radeonsi building errors:

In file included from external/mesa/src/gallium/drivers/radeonsi/si_blit.c:24:
In file included from external/mesa/src/gallium/drivers/radeonsi/si_pipe.h:29:
In file included from external/mesa/src/gallium/drivers/radeonsi/si_shader.h:71:
In file included from external/llvm/include/llvm-c/Core.h:18:
In file included from external/llvm/include/llvm-c/ErrorHandling.h:17:
In file included from external/llvm/include/llvm-c/Types.h:17:
external/llvm/include/llvm/Support/DataTypes.h:49:3: error: "Must #define __STDC_LIMIT_MACROS before #including Support/DataTypes.h"
  ^
external/llvm/include/llvm/Support/DataTypes.h:53:3: error: "Must #define __STDC_CONSTANT_MACROS before "         "#including Support/DataTypes.h"
  ^
2 errors generated.

[Emil Velikov: add inline comment about the defines]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>