If you use GNAT, you can use GPR files for the project. In there you can change the filename for specific packages, for example:
for Specification (Rectangular_Form) use "Rectangular_Method_1.ads";
for Implementation (Rectangular_Form) use "Rectangular_Method_1.adb";
you can even set this depending on an environment variable.
If your spec files all should look the same, you can use a Rectangular_Form.ads and only use the Implementation line from above.
An example GPR file could look like this:
project Example is
type Methods is ("normal", "something_else");
Method : Methods := external ("METHOD", "normal");
package Naming is
case Method is
when "normal" =>
for Implementation ("Example") use "example_normal.adb";
when "something_else" =>
for Implementation ("Example") use "example_something.adb";
end case;
end Naming;
end Example;
Then, you can use gnatmake -P example.gpr to compile it depending on your METHOD variable, or using a -XMETHOD=... parameter for gnatmake or just use the provided default value.
The example_*.adb should all contain the body of the package Example, not Example_Normal, etc..
