@@ ?= is deferred

@@ automatically-generated dependecies should include absolute paths
   just like gcc does. Note that it only happens when you pass gcc
   absolute paths in -I so I should watch out!

@@ order-only dependencies allow for elegant creation of a directory
   before build.

@@ need to try generation of headers in out_dir. 

@@ maybe think about shared library versioning. This could be done 
   transparently at installation time.

%% I should use some form of include function for -include for
   consistency.

@@ linker accepts -O options and can opimize.

@@ use suffix (.make, .cxx) to specify file type - not as a mere
   separator. Can I use '-' as separator? Should I use '_' as separator?

@@ read lisp ASDF manual.

@@ Use $(eval foo:=bar) in rule's command to set variable only if (and after) 
   the rule is in use.
   
%% Files in import directory are called configuration. They are not quite
   that. Maybe a better name would be stub?

@@ For some (all? SUS?) platforms inclusion of .so in .l is redundant 
   because .so is already linked against them.
   
@@ Need to continue cleaning if some files do not exist (rm -f ?).