Parse constants like luajit-ffi

Hello !
Nice work !
While doing some tests with existing luajit-ffi files I noticed that cffi do not understand this declarations as constants like enums:
```
/* kind of structural variable: */
static const int GLP_CV=  1;  /* continuous variable */
static const int GLP_IV = 2;  /* integer variable */
static const int GLP_BV=  3;  /* binary variable */
```
Cheers !