Move symbol exporting .def file rule to bin.def module, add support for MinGW

The bin.def module is automatically loaded by the c and cxx modules for
the *-win32-msvc target architecture. This allows automatically exporting
all symbols for all Windows targets using the following setup (showing
for cxx in this example):

lib{foo}: libul{foo}: {hxx cxx}{**} ...

lib{foo}: def{foo}: include = ($cxx.target.system == 'win32-msvc')
def{foo}: libul{foo}

if ($cxx.target.system == 'mingw32')
  cxx.loptions += -Wl,--export-all-symbols

That is, we use the .def file generation for MSVC and the built-in support
(--export-all-symbols) for MinGW.

But it is also possible to use the .def file generation for MinGW. In this
case we need to explicitly load the bin.def module (which should be done
after loading c or cxx) and using the following setup:

using bin.def  # In root.build.

lib{foo}: libul{foo}: {hxx cxx}{**} ...

lib{foo}: def{foo}: include = ($cxx.target.class == 'windows')
def{foo}: libul{foo}

1 files changed, 4 insertions, 2 deletions

diff --git a/libbuild2/cc/init.cxx b/libbuild2/cc/init.cxx
index eae6d6d..f201d79 100644
--- a/libbuild2/cc/init.cxx
+++ b/libbuild2/cc/init.cxx
@@ -340,15 +340,17 @@ namespace build2
         }
       }
 
-      // Load bin.*.config for bin.* modules we may need (see core_init()
-      // below).
+      // Load bin.* modules we may need (see core_init() below).
       //
       const string& tsys (cast<string> (rs["cc.target.system"]));
 
       load_module (rs, rs, "bin.ar.config", loc);
 
       if (tsys == "win32-msvc")
+      {
         load_module (rs, rs, "bin.ld.config", loc);
+        load_module (rs, rs, "bin.def", loc);
+      }
 
       if (tsys == "mingw32")
         load_module (rs, rs, "bin.rc.config", loc);