It was a massive pain in the butt in that time. Just try to intregrate a "standard library" like libjpeg or libpng into your cross-platform project. That's exactly why everybody has switched over to stb_image.h to quickly load image data without hassle.
Easy, have your "load/save_image" calls the OS provided APIs for image handling, while on UNIX assume the libjpeg-dev or libpng-dev are already installed via the OS package system, have the makefile targets adjusted as needed per platform.
Nowadays, use cmake + conan/vcpkg as alternative as well.
...and with all this you just made everything a thousand times harder (and more brittle) than it needs to be. Integrating stb_image.h is literally just a single #include statement, no matter the platform, build system or dependency manager (if any).
Other than having someone keep track of it, across all projects it gets included, even if it gets vendored somewhere there is the issue with symbol duplication across binary libraries used across teams.
Also it is a toy library in size, then there are those that can be used to stress test compilers.
