These files are thin wrappers for compiling some Ada source code depending on The GNAT Library, to be linked with drake.
If gnatmake is used in the target project, pass this directory to the compiler by the switch -I or the enviromment variable ADA_INCLUDE_PATH.
If gprbuild is used in the target project, at first, run ./configure --RTS=${rts-drake}. (Replace ${rts-drake} to the directory drake is installed.)
Two files named gnat.gpr and c.gpr will be generated.
Next, insert with "gnat.gpr"; into any one of the .gpr file included in the target project.
At last, pass this directory containing gnat.gpr to the gprbuild by the switch -aP or the enviromment variable GPR_PROJECT_PATH.
gnat.gpr indicates this directory. c.gpr indicates the translated headers (c-*.ads) and is needed to make gprbuild understood that the translated headers should not be recompiled.
Of course you can directly copy these files into the directory of the target project.
Many features are unimplemented and impossible to emulate perfectly.