fix for error building head shared on linux

[incompetentcoder]

Hi, trying to build on linux from git head with shared on errors out on shapes.c:(.text+0x20eb): undefined reference to `stbrp_init_target'.
After taking a look at stb documentation I simply added #define STB_RECT_PACK_IMPLEMENTATION at the top of shapes.c and it built and works correctly for me.

[nullstare]

Thanks for the info. I rarely link with shared so I have missed this. I have seem to have "fixed" it for now for just checking if building SHARED and defining STB_RECT_PACK_IMPLEMENTATION. Better would have been to check STB_RECT_PACK_IMPLEMENTATION define but since raylib rtext.c is not doing this it wont help. Not sure how to make a proper fix at this point.

Personally I'm not a huge fan of this sort of hacky single header libraries since they just seem to cause more problems than they supposedly fix.