Remove the depends on zlib

On Ubuntu 14.04 in order to prepare appimages, we should use
the system zlib, not our home-built one

3rdparty/ext_exiv2/CMakeLists.txt

@@ -12,5 +12,5 @@ ExternalProject_Add( ext_exiv2
 	CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${PREFIX_ext_exiv2} -DCMAKE_BUILD_TYPE=${GLOBAL_BUILD_TYPE} ${GLOBAL_PROFILE} -DEXIV2_ENABLE_BUILD_SAMPLES=OFF -DEXIV2_ENABLE_BUILD_PO=OFF -DEXIV2_ENABLE_NLS=OFF -DICONV_INCLUDE_DIR=${PREFIX_ext_exiv2}/include
 
 	UPDATE_COMMAND ""
-	DEPENDS ext_iconv ext_zlib ext_expat
+	DEPENDS ext_iconv ext_expat
 )

3rdparty/ext_openexr/CMakeLists.txt

@@ -26,6 +26,6 @@ ExternalProject_Add(ext_openexr
         CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${EXTPREFIX_openexr} -DILMBASE_PACKAGE_PREFIX=${EXTPREFIX_openexr} -DCMAKE_BUILD_TYPE=${GLOBAL_BUILD_TYPE} ${GLOBAL_PROFILE} -DNAMESPACE_VERSIONING=OFF
 
         UPDATE_COMMAND ""
-        DEPENDS ext_ilmbase ext_zlib
+        DEPENDS ext_ilmbase
 )
 endif()

3rdparty/ext_png/CMakeLists.txt

@@ -8,5 +8,4 @@ ExternalProject_Add( ext_png
 	CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${PREFIX_ext_png} -DCMAKE_BUILD_TYPE=${GLOBAL_BUILD_TYPE} ${GLOBAL_PROFILE} -DPNG_TESTS=OFF
         
 	UPDATE_COMMAND ""
-	DEPENDS ext_zlib
 )

3rdparty/ext_tiff/CMakeLists.txt

@@ -8,5 +8,4 @@ ExternalProject_Add( ext_tiff
     CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${EXTPREFIX_tiff} -DCMAKE_BUILD_TYPE=${GLOBAL_BUILD_TYPE} ${GLOBAL_PROFILE}
 	
     UPDATE_COMMAND ""
-	DEPENDS ext_zlib ext_jpeg
 )