Support WINDOWS_EXPORT_ALL_SYMBOLS with `.def` files

The `WINDOWS_EXPORT_ALL_SYMBOLS` target property exports all symbols
found in object files explicitly given to the linker.  However, the
linker may also find additional symbols in dependencies and copy them
into the linked binary (e.g. from `msvcrt.lib`).  Provide a way to
export an explicit list of such symbols by adding a `.def` file as a
source file.

Fixes: #16473

1 files changed, 7 insertions, 0 deletions

diff --git a/Help/prop_tgt/WINDOWS_EXPORT_ALL_SYMBOLS.rst b/Help/prop_tgt/WINDOWS_EXPORT_ALL_SYMBOLS.rst
index 06c3e6d..86711bf 100644
--- a/Help/prop_tgt/WINDOWS_EXPORT_ALL_SYMBOLS.rst
+++ b/Help/prop_tgt/WINDOWS_EXPORT_ALL_SYMBOLS.rst
@@ -14,6 +14,13 @@ be automatically exported and imported by callers.  This simplifies porting
 projects to Windows by reducing the need for explicit ``dllexport`` markup,
 even in ``C++`` classes.
 
+When this property is enabled, zero or more ``.def`` files may also be
+specified as source files of the target.  The exports named by these files
+will be merged with those detected from the object files to generate a
+single module definition file to be passed to the linker.  This can be
+used to export symbols from a ``.dll`` that are not in any of its object
+files but are added by the linker from dependencies (e.g. ``msvcrt.lib``).
+
 This property is initialized by the value of
 the :variable:`CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS` variable if it is set
 when a target is created.