BUG: missing libpng target for Linux

CMake/External_libpng.cmake

@@ -2,7 +2,7 @@ message(STATUS "Setup libpng...")
 
 set(proj PNG)
 
-set(DEFAULT_USE_SYSTEM_PNG  OFF)
+set(DEFAULT_USE_SYSTEM_PNG  ON)
 
 option(USE_SYSTEM_PNG "  Use a system build of libtiff." ${DEFAULT_USE_SYSTEM_TIFF})
 mark_as_advanced(USE_SYSTEM_PNG)
@@ -18,16 +18,18 @@ else()
     set(PNG_SB_ZLIB_CONFIG)
   else()
     if(MSVC)
-        set(PNG_SB_ZLIB_CONFIG
-            -DZLIB_INCLUDE_DIR:PATH=${CMAKE_INSTALL_PREFIX}/include
-            -DZLIB_LIBRARY:FILEPATH=${CMAKE_INSTALL_PREFIX}/lib/zlib.lib)
-        list(APPEND ${proj}_DEPENDENCIES ZLIB)
+      set(PNG_SB_ZLIB_CONFIG
+        -DZLIB_INCLUDE_DIR:PATH=${CMAKE_INSTALL_PREFIX}/include
+        -DZLIB_LIBRARY:FILEPATH=${CMAKE_INSTALL_PREFIX}/lib/zlib.lib)
+    else(UNIX)
+      set(PNG_SB_ZLIB_CONFIG
+        -DZLIB_INCLUDE_DIR:PATH=${CMAKE_INSTALL_PREFIX}/include
+        -DZLIB_LIBRARY:FILEPATH=${CMAKE_INSTALL_PREFIX}/lib/libz.so)
     endif()
+    list(APPEND ${proj}_DEPENDENCIES ZLIB)
   endif()
   
-  if(MSVC)
-  
-     ExternalProject_Add(${proj}
+    ExternalProject_Add(${proj}
       PREFIX ${proj}
       URL "http://sourceforge.net/projects/libpng/files/libpng16/1.6.16/lpng1616.zip/download"
       URL_MD5 c90c9587c9a5c735327fb3f6900f6b03
@@ -35,16 +37,11 @@ else()
       INSTALL_DIR ${CMAKE_INSTALL_PREFIX}
       DEPENDS ${${proj}_DEPENDENCIES}
       CMAKE_CACHE_ARGS
-            -DCMAKE_BUILD_TYPE:STRING=Release
-            -DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_INSTALL_PREFIX}
-            ${PNG_SB_ZLIB_CONFIG}
+        -DCMAKE_BUILD_TYPE:STRING=Release
+        -DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_INSTALL_PREFIX}
+        ${PNG_SB_ZLIB_CONFIG}
       CMAKE_COMMAND ${SB_CMAKE_COMMAND}
     )
 
-  else(UNIX)
-    #TODO
-    
-  endif()
-  
   message(STATUS "  Using libpng SuperBuild version")
 endif()