Error compiling library in Arduino 1.6.4

Error when compiling:

```
/Users/tguidon/Documents/Arduino/libraries/HT1632/glcdfont.c:15:23: error: variable 'font' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
static unsigned char font[] PROGMEM = {
^
Error compiling
```

Sketch compiles fine on Arduino 1.0.5

Start by reproducing the compilation failure with Arduino 1.6.4 using the library and inspect glcdfont.c around line 15, where the font declaration is reported. Compare the behavior with Arduino 1.0.5; done means the library compiles successfully on the newer Arduino version without regressing the older case.

Written by the indexing model from the issue text.