meson, plug-ins: further fix the no-pkg-config test for libmng.

As suggested by jeremyd2019 and Biswa96 of MSYS2 project, defining the
MNG_USE_DLL macro would trigger libmng header code to fix the calling
convention. This way, even the old detection code now works with Windows
32-bit.

The reason why we still stick to this old detection code is that the
pkg-config is likely not everywhere (e.g. in Debian package libmng-dev,
there is no `.pc` file). So the pkg-config test is good but we still
definitely need to keep our fallback more old-school test for this
dependency.

See this comment in particular:
https://github.com/msys2/MINGW-packages/issues/11136#issuecomment-1084627263

Note: I am unsure yet how to apply the same trick on the autotools test
because AC_CHECK_LIB() would allow us to tweak the cflags to define a
macro, but we also need to include libmng.h header, otherwise the
calling convention tweak is not run. It doesn't look to be feasible with
the autoconf macro, or at least I haven't found how yet.

meson.build

@@ -661,8 +661,14 @@ MIMEtypes += [ 'image/png', 'image/x-icon']
 libmng = dependency('libmng', required: get_option('mng'))
 
 if not libmng.found()
-  libmng = cc.find_library('mng', required: get_option('mng'))
-  if libmng.found() and not cc.has_function('mng_create', dependencies: libmng)
+  libmng = cc.find_library('mng', required: get_option('mng'),)
+
+  mng_test_prefix = ''
+  if platform_windows
+    mng_test_prefix = '#define MNG_USE_DLL\n#include <libmng.h>'
+  endif
+  if libmng.found() and not cc.has_function('mng_create', dependencies: libmng,
+                                            prefix: mng_test_prefix)
     libmng = no_dep
   endif
 endif

plug-ins/common/meson.build

@@ -111,8 +111,13 @@ if libjxl.found() and libjxl_threads.found()
 endif
 
 if libmng.found()
+  mng_cflags = []
+  if platform_windows
+    mng_cflags = [ '-DMNG_USE_DLL' ]
+  endif
   common_plugins_list += { 'name': 'file-mng',
-    'deps': [ gtk3, gegl, libmng, libpng, ],
+                           'deps': [ gtk3, gegl, libmng, libpng, ],
+                           'cflags': mng_cflags,
   }
 endif