SuperBuild: add libpng to system checkup

SuperBuild/CMake/SystemCheckup/CMakeLists.txt

@@ -140,7 +140,7 @@ SB_CHECKUP_WRITE_RESULT(CURL)
 # GDAL
 SB_CHECKUP_FIND_PACKAGE(GDAL)
 if(NOT _SB_CHECKUP_GDAL_VERSION)
-  SB_CHECKUP_FIND_VERSIONS(GDAL 
+  SB_CHECKUP_FIND_VERSIONS(GDAL
                            gdal_version.h
                            GDAL_VERSION_MAJOR
                            GDAL_VERSION_MINOR
@@ -159,7 +159,7 @@ SB_CHECKUP_WRITE_RESULT(ITK)
 # libkml
 SB_CHECKUP_FIND_PACKAGE(LibKML)
 if(NOT _SB_CHECKUP_LIBKML_VERSION)
-  SB_CHECKUP_FIND_VERSIONS(LibKML 
+  SB_CHECKUP_FIND_VERSIONS(LibKML
                            kml/base/version.h
                            LIBKML_MAJOR_VERSION
                            LIBKML_MINOR_VERSION
@@ -204,6 +204,10 @@ SB_CHECKUP_WRITE_RESULT(OpenThreads)
 SB_CHECKUP_FIND_PACKAGE(Ossim)
 SB_CHECKUP_WRITE_RESULT(Ossim)
 
+# PNG
+SB_CHECKUP_FIND_PACKAGE(PNG)
+SB_CHECKUP_WRITE_RESULT(PNG)
+
 # Qt4
 SB_CHECKUP_FIND_PACKAGE(Qt4)
 if(_SB_CHECKUP_QT4_FOUND)