Hi, first of all, thanks for the hard work you do on Libretro!
In commit 8f1ee60, a change was introduced to include/libchdr/cdrom.h and include/libchdr/chd.h to import the header libchdr/chdconfig.h, but libchdr/chdconfig.h is not present in the repository. Commenting out that include is good enough to compile as it seems that header is unused.