I think this is missing half of the problem, that is connecting the package with the build system. I know there is no "standard" build system for C, but still. Go for instance does both; once you "go get" a package, it not only downloads and pre-builds it, but makes sure that it is available for transparent usage in your program.
I personally bias towards CMake, so I would appreciate a C package manager which also makes sure that the package is included in my CMakeList.txt somehow.
I can't see how it's simpler, because then you need to write N*M bridges/connectors/plugins to let N package managers interact with M build systems. And what if the library you download is meant to be built with a build system which is different from the one you chose for your application? What if you then want to globally turn on debugging or change a compilation flag, or a preprocessor define to enable/disable a feature?
I don't know of any language-specific package manager that doesn't also make the package immediately available to the programming environment without further fiddling. Solving half of the problem... well, it's a half solution :)
I personally bias towards CMake, so I would appreciate a C package manager which also makes sure that the package is included in my CMakeList.txt somehow.