BUG: Initializes CMAKE_DEBUG_POSTFIX to be empty

When GoogleTest third party module was updated (commit 5be14a5),
CMAKE_DEBUG_POSTFIX was set to 'd', which means that the ITK library
filenames will end with 'd'. This is not how ITK libraries used to
be named. To keep backward compatibility, we remove this postfix.

Change-Id: I802dcff5c6210692d64c38668b59c19c3c823662

CMakeLists.txt

@@ -210,6 +210,10 @@ set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${ITK_REQUIRED_LINK_
 if(NOT CMAKE_POSITION_INDEPENDENT_CODE)
   set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 endif()
+# Because GoogleTest sets CMAKE_DEBUG_POSTFIX CMake CACHE variable to "d" when it is
+# built in debug, we preemptively set it to an empty string to avoid having a postfix
+# added to the ITK library names.
+set(CMAKE_DEBUG_POSTFIX "" CACHE STRING "Generate debug library name with a postfix.")
 
 # Setup build locations.
 if(NOT CMAKE_RUNTIME_OUTPUT_DIRECTORY)