pkg-config is the defacto standard for discovering build flags and library names on Linux, so it woulf be beneficial to have giflib install one.
The file would go to /usr/lib/pkgconfig/giflib.pc and will contain:
prefix=/usr
exec_prefix=/usr
libdir=/usr/lib
includedir=/usr/include
Name: giflib
Description: Loads GIF files
Version: 5.2.1
Libs: -L${libdir} -lgif
Cflags: -I${includedir}
Because this file contains system paths, I think I would prefere to leave this detail to distribution packages.
Your Makefile also contains system paths, so that's not much of a reason. All other projects that do include a pkg-config file do so in the main codebase. Distribution packages generally refuse to the pkg-config file because they consider it to be upstream responsibility. My request to add a .pc file to the giflib package on Arch was denied for that reason.