cmake: added option for libpng

CMakeLists.txt

@@ -95,10 +95,13 @@ if(WITH_OPENMP)
   endif()
 endif()
 
-#find_package(PNG)
-if (PNG_FOUND)
-  INCLUDE_DIRECTORIES(${PNG_INCLUDE_DIRS})
-  ADD_DEFINITIONS( "-DNICE_USELIB_PNG")
+NICE_OPTION(WITH_PNG "Build with libPNG support" OFF)
+if(WITH_PNG)
+    find_package(PNG)
+    if (PNG_FOUND)
+      INCLUDE_DIRECTORIES(${PNG_INCLUDE_DIRS})
+      ADD_DEFINITIONS( "-DNICE_USELIB_PNG")
+    endif()
 endif()
 
 NICE_OPTION(WITH_MATIO "Build with Matio and HDF5 support" OFF)
@@ -244,4 +247,4 @@ SET(CPACK_GENERATOR "RPM")
 SET(CPACK_DEBIAN_PACKAGE_MAINTAINER "Johannes R.") #required
 SET(CPACK_PACKAGE_NAME "NICE New Image Library")
 SET(CPACK_PACKAGE_VENDOR "Computer Vision Group Jena, University of Jena")
-INCLUDE(CPack)
+INCLUDE(CPack)