Ok, so the Form parameter is implementation defined; this was to allow the implementations the 'wriggle room' to interface with the host-system.
For GNAT, these two pieces of documentation are instructive:
https://docs.adacore.com/live/wave/gnat_rm/html/gnat_rm/gnat...
https://gcc.gnu.org/onlinedocs/gcc-4.9.1/gnat_rm/FORM-String...
(This second one is older documentation, but illustrates how platform-specific Form parameters could be used.)
Ada.Text_IO.Create (
File => File,
Mode => Ada.Text_IO.Out_File,
Name => "test.txt",
Form => "shared=no"
);
The "maze of alternative package bodies and accompanying conditional compilation logic" is an artifact of C's approach to 'portability' using the preprocessor. Typically, the conditionality should be stable once you abstract it (using the compiler's project-management to select the correct body for a particular configuration) -- As a stupidly trivial example, consider the path separator, for the specification you could have:
Package Dependency is
Package OS is
Function Separator return String;
End OS;
End Dependency;
-- ...
Package Dependency is
Package body OS is separate;
End Dependency;
-- Windows
separate (Dependency)
Package OS is
Function Separator return String is ("\");
End OS;
-- Classic Mac
separate (Dependency)
Package OS is
Function Separator return String is (":");
End OS;
-- VMS
separate (Dependency)
Package OS is
Function Separator return String is (".");
End OS;
-- UNIX-like
separate (Dependency)
Package OS is
Function Separator return String is ("/");
End OS;
Then in your the rest of your program, you program against the abstraction of DEPENDENCY.OS (and whatever other dependencies you have, likewise), and thus separate out the implementation dependency.