libexif (and some other external libraries) are searched in 2 approaches in CamlImages:
find exif-data.h in include directories.
pkg-config is the main way to configure libexif automatically in CamlImages. I presume pkg-config fails in your environment. I recommend to check pkg-config is installed and libexif is installed with enabling pkg-config feature.
Finding exif-data.h in the include directories is just a failover when pkg-config fails.I am not going to add any clever way to find out include files automatically, since I have no way to find out those directories and test them for the environments I do not have.
However, there is a manual way already. INSTALL.txt says how to add your include directories:
pkg-config is fine, that stuff just isn't included in 4.1.0 tarball yet which is what I was fixing in Fedora. However I still suggest fixing the fallback to check for libexif/exif-data.h instead of exif-data.h because if exif headers are not installed in "libexif/" dir, build will fail anyway if exif-data.h is found in system include dirs because src/exif_c.c #includes <libexif/*.h>.