Use git or http protocol to download dcmtk

DCMTK servers now support git protocol, use it by default.
Closes #33

CMakeExternals/DCMTK.cmake

@@ -37,7 +37,7 @@ IF(${add_project})
       SOURCE_DIR ${CMAKE_BINARY_DIR}/${proj}
       BINARY_DIR ${proj}-build
       PREFIX ${proj}${ep_suffix}
-      GIT_REPOSITORY "http://git.dcmtk.org/dcmtk.git"
+      GIT_REPOSITORY "${git_protocol}://git.dcmtk.org/dcmtk.git"
       GIT_TAG ${revision_tag}
       CMAKE_GENERATOR ${gen}
       UPDATE_COMMAND ""