Implement initial support for library versioning

Currently we only support platform-independent versions that get appended
to the library name. The magic incantation is this:

lib{foo}: bin.lib.version = @-1.2

This will produce libfoo-1.2.so, libfoo-1.2.dll, etc.

In the future we will support things like this:

lib{foo}: bin.lib.version = linux@1.2.3 freebsd@1.2 windows@1.2

1 files changed, 10 insertions, 7 deletions

diff --git a/build2/target.cxx b/build2/target.cxx
index 7a33dbc..8541deb 100644
--- a/build2/target.cxx
+++ b/build2/target.cxx
@@ -351,17 +351,20 @@ namespace build2
   const path& path_target::
   derive_path (const char* de, const char* np, const char* ns)
   {
-    string n;
+    path_type p (dir);
 
-    if (np != nullptr)
-      n += np;
-
-    n += name;
+    if (np == nullptr)
+      p /= name;
+    else
+    {
+      p /= np;
+      p += name;
+    }
 
     if (ns != nullptr)
-      n += ns;
+      p += ns;
 
-    return derive_path (dir / path_type (move (n)), de);
+    return derive_path (move (p), de);
   }
 
   const path& path_target::