Make MSVC symbol export/import work

tests/packages/sharedlib-in-package/mypkg/examplelib.c

@@ -2,6 +2,8 @@
 //
 // SPDX-License-Identifier: MIT
 
-int sum(int a, int b) {
+#include "mypkg_dll.h"
+
+MYPKG_DLL int sum(int a, int b) {
 	return a + b;
 }

tests/packages/sharedlib-in-package/mypkg/examplelib.h

@@ -2,4 +2,6 @@
 //
 // SPDX-License-Identifier: MIT
 
-int sum(int a, int b);
+#include "mypkg_dll.h"
+
+MYPKG_DLL int sum(int a, int b);

tests/packages/sharedlib-in-package/mypkg/meson.build

@@ -2,17 +2,31 @@
 #
 # SPDX-License-Identifier: MIT
 
+if meson.get_compiler('c').get_id() in ['msvc', 'clang-cl', 'intel-cl']
+  export_dll_args = ['-DMYPKG_DLL_EXPORTS']
+  import_dll_args = ['-DMYPKG_DLL_IMPORTS']
+else
+  export_dll_args = []
+  import_dll_args = []
+endif
+
 example_lib = shared_library(
     'examplelib',
     'examplelib.c',
+    c_args: export_dll_args,
     install: true,
     install_dir: py.get_install_dir() / 'mypkg',
 )
 
+example_lib_dep = declare_dependency(
+  compile_args: import_dll_args,
+  link_with: example_lib,
+)
+
 py.extension_module(
     '_example',
     '_examplemod.c',
-    link_with: example_lib,
+    dependencies: example_lib_dep,
     install: true,
     subdir: 'mypkg',
     install_rpath: '$ORIGIN',

tests/packages/sharedlib-in-package/mypkg/mypkg_dll.h

@@ -0,0 +1,19 @@
+#pragma once
+
+// MYPKG_DLL
+// inspired by https://github.com/abseil/abseil-cpp/blob/20240116.2/absl/base/config.h#L736-L753
+// and https://github.com/scipy/scipy/blob/9ded83b51099eee745418ccbb30196db96c81f3f/scipy/_build_utils/src/scipy_dll.h
+//
+// When building the `examplelib` DLL, this macro expands to `__declspec(dllexport)`
+// so we can annotate symbols appropriately as being exported. When used in
+// headers consuming a DLL, this macro expands to `__declspec(dllimport)` so
+// that consumers know the symbol is defined inside the DLL. In all other cases,
+// the macro expands to nothing.
+// Note: MYPKG_DLL_{EX,IM}PORTS are set in mypkg/meson.build
+#if defined(MYPKG_DLL_EXPORTS)
+    #define MYPKG_DLL __declspec(dllexport)
+#elif defined(MYPKG_DLL_IMPORTS)
+    #define MYPKG_DLL __declspec(dllimport)
+#else
+    #define MYPKG_DLL
+#endif