undefined reference to `adainit'

I must admit that I'm not very experienced with Ada. I'm working with an 
interface since I need to be able to call a library written in Ada from 
..NET. I made a small project and compiled it as follows:

gnatmake -c api
gnatbind -n api
gnatdll -d api.dll api.ali

Now, everything works fine. I'm able to call the library from a C# .NET 
program. However, if I create a GPS project file an try to compile, I get 
the following error:

i:\temp\adatest\adadll\obj\api.o:api.adb:(.text+0x38): undefined reference

to `adainit'
i:\temp\adatest\adadll\obj\api.o:api.adb:(.text+0x57): undefined reference

to `adafinal'
gnatmake: gcc execution error

Does anyone know what to do? The GPS project file look like this:

project Api is

   for Library_Name use "api";
   for Library_Dir use "dll";
   for Library_Ali_Dir use "ali";
   for Library_Kind use "dynamic";
   for Languages use ("Ada");
   for Object_Dir use "obj";
   for Library_Interface use ("API");
   for Library_Auto_Init use "False";
   for Library_Src_Dir use "dll";

   package Compiler is
      for Default_Switches ("Ada") use ("-gnat95", "-gnatv", "-lgnat");
   end Compiler;

   package Linker is
      for Linker_Options use ("-lgnat");
   end Linker;

end Api;