[libpng15] Added MINGW support to CMakeLists.txt

CHANGES

diff --git a/CHANGES b/CHANGES
index 763efca..f44c0f6 100644
--- a/CHANGES
+++ b/CHANGES
@@ -3749,6 +3749,7 @@
   Validate time settings passed to pngset() and png_convert_to_rfc1123()
     (Frank Busse).
   Allow row_stride==0 to indicate default stride in simplified API
+  Added MINGW support to CMakeLists.txt
 
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