diff --git a/gnat/spawn.gpr b/gnat/spawn.gpr
index 9d814be..f266eb1 100644
--- a/gnat/spawn.gpr
+++ b/gnat/spawn.gpr
@@ -9,7 +9,7 @@ with "../config/spawn_config.gpr";
 library project Spawn is
 
    type OS_API_Kind is ("unix", "osx", "Windows_NT");
-   OS_API : OS_API_Kind := external ("OS", "unix");
+   OS_API : OS_API_Kind := external ("SPAWN_OS", external ("OS", "unix"));
 
    type Library_Kind is ("static", "static-pic", "relocatable");
    Library_Type : Library_Kind :=
diff --git a/gnat/spawn_glib.gpr b/gnat/spawn_glib.gpr
index f12701c..c438df6 100644
--- a/gnat/spawn_glib.gpr
+++ b/gnat/spawn_glib.gpr
@@ -10,7 +10,7 @@ with "../config/spawn_config.gpr";
 library project Spawn_Glib is
 
    type OS_API_Kind is ("unix", "osx", "Windows_NT");
-   OS_API : OS_API_Kind := external ("OS", "unix");
+   OS_API : OS_API_Kind := external ("SPAWN_OS", external ("OS", "unix"));
 
    type Library_Kind is ("static", "static-pic", "relocatable");
    Library_Type : Library_Kind :=