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, 6 insertions, 0 deletions

diff --git a/libbuild2/bin/init.hxx b/libbuild2/bin/init.hxx
index 6b0db27..4eb0f10 100644
--- a/libbuild2/bin/init.hxx
+++ b/libbuild2/bin/init.hxx
@@ -25,14 +25,20 @@ namespace build2
     //                    rules.
     // `bin.ar.config` -- loads bin.config and registers/sets more variables.
     // `bin.ar`        -- loads bin and bin.ar.config.
+    //
     // `bin.ld.config` -- loads bin.config and registers/sets more variables.
     // `bin.ld`        -- loads bin and bin.ld.config and registers more
     //                    target types for msvc.
+    //
     // `bin.rc.config` -- loads bin.config and registers/sets more variables.
     // `bin.rc`        -- loads bin and bin.rc.config.
+    //
     // `bin.nm.config` -- loads bin.config and registers/sets more variables.
     // `bin.nm`        -- loads bin and bin.nm.config.
     //
+    // `bin.def`       -- loads bin, bin.nm.config unless using MSVC link.exe,
+    //                    and registers the .def file generation rule.
+    //
     extern "C" LIBBUILD2_BIN_SYMEXPORT const module_functions*
     build2_bin_load ();
   }