Add extra support for symbol exporting in modularized projects

Since modules don't see each other's macros, we can use a single, keyword-
like macro for dll-exporting that is managed by the build system (so no
need for an "export" header). For example:

cxx.features.symexport = true

export __symexport void f ();

1 files changed, 32 insertions, 0 deletions

diff --git a/tests/cc/modules/testscript b/tests/cc/modules/testscript
index 23f8946..a65bdac 100644
--- a/tests/cc/modules/testscript
+++ b/tests/cc/modules/testscript
@@ -14,6 +14,8 @@ EOI
 +cat <<EOI >=build/root.build
 cxx.std = experimental
 
+cxx.features.symexport = true
+
 # Force modules.
 #
 cxx.features.modules = true
@@ -313,3 +315,33 @@ $* test clean <<EOI
   exe{test}: cxx{driver} {mxx cxx}{core}
   bmie{foo-core}: mxx{foo-core}
   EOI
+
+: symexport
+:
+: Test the __symexport feature.
+:
+cat <<EOI >=core.mxx;
+  #if __cpp_modules >= 201704
+  export
+  #endif
+  module foo.core;
+
+  export __symexport int f (int);
+
+  __symexport int g_impl (int i) {return i - 1;}
+  export __symexport inline int g (int i) {return g_impl (i);}
+  EOI
+ln -s ../core.cxx core-f.cxx;
+cat <<EOI >=core-g.cxx;
+  module foo.core;
+  int g_impl (int i) {return i - 1;}
+  EOI
+cat <<EOI >=driver.cxx;
+  import foo.core;
+  int main (int argc, char*[]) {return f (argc) + g (argc);}
+  EOI
+$* test clean <<EOI
+  ./: lib{foo} exe{test} # Full build.
+  exe{test}: cxx{driver} lib{foo}
+  lib{foo}: mxx{core} cxx{core-f} # core-g @@ VC
+  EOI