If we want to use import <iostream>; instead of old style #include
<iostream> we need to "precompile" the iostream header file. We can do
it manually by simply invoking g++ with:
g++ -xc++-system-header -std=c++20 -fmodules-ts iostream
But this needs a manually created set of rules for all used system
header files for gnu make with all the dependencies pointing to the
gcm.cache directory structure.
In case of old style #include directives, we simply have the -M or -MM
flags to generate such dependencies for make automatically.
Q: How we can get such dependencies from g++ for imported modules
automatically to set up the rules for gnu make?
The target is quite simple: Get all needed .gcm files precompiled before
the build system needs these files for import.
