Remove empty build command and set install command empty. Thus is cannot happen

that the dcmtk libraries are installed to some unintended location.
Setting dcmtk_dir to dcmtk-build directory

CMakeExternals/DCMTK.cmake

@@ -52,7 +52,7 @@ if(NOT DEFINED DCMTK_DIR AND NOT ${CMAKE_PROJECT_NAME}_USE_SYSTEM_${proj})
     BINARY_DIR ${proj}-build
     PREFIX ${proj}${ep_suffix}
     ${location_args}
-    BUILD_COMMAND ""
+    INSTALL_COMMAND ""
     CMAKE_ARGS
       -DDCMTK_INSTALL_BINDIR:STRING=bin/${CMAKE_CFG_INTDIR}
       -DDCMTK_INSTALL_LIBDIR:STRING=lib/${CMAKE_CFG_INTDIR}
@@ -72,7 +72,7 @@ if(NOT DEFINED DCMTK_DIR AND NOT ${CMAKE_PROJECT_NAME}_USE_SYSTEM_${proj})
     DEPENDS
       ${${proj}_DEPENDENCIES}
     )
-  set(DCMTK_DIR ${ep_install_dir})
+  set(DCMTK_DIR ${CMAKE_CURRENT_BINARY_DIR}/${proj}-build)
 
 else()
   ExternalProject_Add_Empty(${proj} DEPENDS ${${proj}_DEPENDENCIES})