Redesign obj to exe/lib mapping

Specifically:

* objso{} and libso{} target types have been renamed to objs{} and libs{}

* obje{} has been added (so now we have obje{}, obja{}, and objs{})

* obje{} is now used for building exe{}

* object file extensions have been changed to use "hierarchical extensions"
  that reflect the extension of the corresponding exe/lib target (instead of
  the -so suffix we used), specifically:

  obje{}: foo.o, (UNIX), foo.exe.o (MinGW), foo.exe.obj (Windows)
  obja{}: foo.a.o (UNIX, MinGW), foo.lib.obj (Windows)
  objs{}: foo.so.o (UNIX), foo.dylib.o (Darwin), foo.dll.o (MinGW),
          foo.dll.obj (Windows)

1 files changed, 1 insertions, 1 deletions

diff --git a/build2/target b/build2/target
index 955c2f9..28753df 100644
--- a/build2/target
+++ b/build2/target
@@ -192,7 +192,7 @@ namespace build2
     // is an explicit mechanism for discovering the group's members.
     //
     // However, sometimes, we may want to create a group on the fly out of a
-    // normal target type. For example, we have the libso{} target type. But
+    // normal target type. For example, we have the libs{} target type. But
     // on Windows a shared library consist of (at least) two files: the import
     // library and the DLL itself. So we somehow need to be able to capture
     // that. One approach would be to imply the presence of the second file.