meson.build: Improve libpng search on MSVC

The build system for libpng for MSVC does not generate a pkg-config file
for us, and CMake support in Meson does not work very well.  So, look
for libpng manually on MSVC builds if depedency discovery did not work
out via pkg-config or the CMake config files.

meson.build

@@ -387,7 +387,24 @@ dep_gtk = dependency('gtk+-2.0', version : '>= 2.16', required : get_option('gtk
 dep_glib = dependency('glib-2.0', required : get_option('gtk'))
 dep_pixman = dependency('pixman-1', required : get_option('gtk'),
                         version : '>= ' + meson.project_version())
-dep_png = dependency('libpng', required : get_option('libpng'))
+
+dep_png = null_dep
+if not get_option('libpng').disabled()
+  dep_png = dependency('libpng', required : false)
+
+  # We need to look for the right library to link to for libpng,
+  # when looking for libpng manually
+  foreach png_ver : [ '16', '15', '14', '13', '12', '10' ]
+    if not dep_png.found()
+      dep_png = cc.find_library('libpng@0@'.format(png_ver), has_headers : ['png.h'], required : false)
+    endif
+  endforeach
+
+  if get_option('libpng').enabled() and not dep_png.found()
+    error('libpng support requested but libpng library not found')
+  endif
+endif
+
 if dep_png.found()
   config.set('HAVE_LIBPNG', 1)
 endif