Line 3: Imports the env from the parent script
Line 5&7: Sets up the C compiler to look in both the current directory and the include directory underneath the main directory in which SCons was started. Note the "#" in the "#include". This is important. That include directory contains a "config.h" file that almost all the C code references and it is not copied over to the build/etc/etc hierarchy.
Line 9: Gambit Scheme input files (sources)
Line 11: Converts the Gambit Scheme input files into C output files (targets) via the GambitCompiler Builder defined in the top-level SConstruct. ie. _gsi.scm becomes _gsi.c. Note that I am saving the targets here for later reference.
Line 15: Make the targets dependent upon libgambc. As the comment says, only the gsi executable is, strictly speaking, dependent upon libgambc. However, the easiest way to pick up the dependencies from _gsi.scm to the lib files is to make the _gsi target depend on the whole library. Yes, it's a bit of a hack which forces libgambc to build first.
Line 17: Gambit requires some extra link information to produce a stand-alone executable. This uses the GambitLinker Builder that was defined in the top-level SConstruct to produce that file. A better example of this is in the gsc SConscript which demonstrates the multiple sources compiling into one target. Here I only have one source.
Line 18: Invoke the normal SCons Program Builder to produce an executable and remember it
Line 19: Not used yet, but I send the reference to the executable back up to the SConstruct for later use, if required.
1 # -*-python-*- 2 3 Import(["env", "libgambc"]) 4 5 cpppath = [".", "#include"] 6 7 env.Replace(CPPPATH=cpppath) 8 9 gambitSourceFiles = ["_gsi.scm"] 10 11 gambitTargetFiles = env.GambitCompiler(gambitSourceFiles) 12 13 # The target files are not strictly dependent upon the entire 14 # libgambc but on some of the files generated during its build 15 env.Depends(gambitTargetFiles, libgambc) 16 17 gsiLinkerFile = env.GambitLinker([gambitTargetFiles]) 18 gsi = env.Program("gsi", [gambitTargetFiles, gsiLinkerFile, libgambc]) 19 Return("gsi")