Thanks for linking to libpipeline; I didn't know about it. The API looks good. However, since it is licensed under the GNU GPL (the initial version was based on code in GNU troff) non-copyleft libraries like ImageMagick are unlikely to adopt it.

If you are looking for a small, standalone, non-copyleft alternative, the closest I can think of is the [exec] command in the Jim Tcl [1] embedded scripting language.

Here is an example (without the error checks you'd have in production code):

  #include "jim.h"

  int main(int argc, char const *argv[])
  {
      Jim_Interp *interp;
      int error;
      Jim_Obj *cmd;

      interp = Jim_CreateInterp();
      Jim_RegisterCoreCommands(interp);
      Jim_InitStaticExtensions(interp);

      // The input redirect below does *not* invoke the POSIX shell. It is handled by Jim Tcl itself.
      cmd = Jim_NewListObj(interp, NULL, 0);
      Jim_ListAppendElement(interp, cmd, Jim_NewStringObj(interp, "exec", -1));
      Jim_ListAppendElement(interp, cmd, Jim_NewStringObj(interp, "awk", -1));
      Jim_ListAppendElement(interp, cmd, Jim_NewStringObj(interp, "1", -1));
      Jim_ListAppendElement(interp, cmd, Jim_NewStringObj(interp, "<", -1));
      Jim_ListAppendElement(interp, cmd, Jim_NewStringObj(interp, "/etc/passwd", -1));

      error = Jim_EvalObj(interp, cmd);
      if (error != JIM_ERR) {
          printf("%s\n", Jim_String(Jim_GetResult(interp)));
      }

      Jim_FreeInterp(interp);
      return error;
  }

While I am a fan of the language and of the "hard and soft layers" approach in general [2], it is a commitment: it requires you to embed the language runtime in your program and learn the basics of the scripting language itself and its C API. The upside is that Jim Tcl's [exec] works on Windows, too (and you get other goodies with Jim like a fast, high quality implementation of strings and hash maps).

Alternatively, you can use the "big" Tcl [3] as a C library. It's larger but more mature and is available in every major Linux distribution.

[1] http://jim.tcl.tk/fossil/doc/trunk/Tcl_shipped.html#_exec

[2] http://c2.com/cgi/wiki?AlternateHardAndSoftLayers

[3] https://tcl.wiki/2074