Handle *.export.libs, distinguish interface and implementation dependencies

A library dependency on another libraries is either "interface" or
"implementation". If it is interface, then everyone who links to this library
should also link to the interface dependency, explicitly. A good example of
an interface dependency is a library API that is called in inline functions.

Interface dependencies of a library should be explicitly listed in the
*.export.libs (where we can also list target names). So the typical usage
will be along these lines:

import int_libs = libfoo%lib{foo}
import int_libs += ...

import imp_libs = libbar%lib{bar}
import imp_libs += ...

lib{baz}: ... $int_libs $imp_libs

lib{baz}: cxx.export.libs = $int_libs

1 files changed, 14 insertions, 0 deletions

diff --git a/build2/utility.ixx b/build2/utility.ixx
index 956a726..36c7a7f 100644
--- a/build2/utility.ixx
+++ b/build2/utility.ixx
@@ -42,6 +42,13 @@ namespace build2
 
   template <typename T>
   inline void
+  append_options (strings& args, T& s, const variable& var)
+  {
+    append_options (args, s[var]);
+  }
+
+  template <typename T>
+  inline void
   append_options (cstrings& args, T& s, const char* var)
   {
     append_options (args, s[var]);
@@ -49,6 +56,13 @@ namespace build2
 
   template <typename T>
   inline void
+  append_options (strings& args, T& s, const char* var)
+  {
+    append_options (args, s[var]);
+  }
+
+  template <typename T>
+  inline void
   hash_options (sha256& csum, T& s, const variable& var)
   {
     hash_options (csum, s[var]);