*nix installation has no pkg-config file

I fixed this on my system by adding a `freetype-gl.pc` file to `/usr/share/pkgconfig`.

The file contains

```
prefix=/usr/local
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${exec_prefix}/include

Name: freetype-gl
Description: Displays text in opengl
Version: 0.1.0
Libs: -L${libdir} -lfreetype-gl
Cflags: -I${includedir}/freetype-gl
```