ENH: Set plural library variables for GDCM system deps

GDCM's CMakeLists.txt guards find_package() calls with checks on
ZLIB_LIBRARIES and EXPAT_LIBRARIES (plural forms). Set these from ITK's
bundled library targets so the guards work correctly.

Author: blowekamp

Modules/ThirdParty/GDCM/src/CMakeLists.txt

@@ -34,6 +34,7 @@ set(GDCM_BUILD_TESTING_DEFAULT OFF)
 set(GDCM_USE_SYSTEM_EXPAT ON CACHE INTERNAL "")
 set(EXPAT_INCLUDE_DIR ${ITKExpat_INCLUDE_DIRS})
 set(EXPAT_LIBRARY ${ITKExpat_LIBRARIES})
+set(EXPAT_LIBRARIES ${ITKExpat_LIBRARIES})
 # OpenJPEG:
 set(GDCM_USE_SYSTEM_OPENJPEG ON CACHE INTERNAL "")
 set(GDCM_USE_OPENJPEG_V2 ON CACHE INTERNAL "")
@@ -42,7 +43,9 @@ set(OPENJPEG_LIBRARIES "ITK::ITKOpenJPEGModule")
 # ZLIB:
 set(GDCM_USE_SYSTEM_ZLIB ON CACHE INTERNAL "")
 set(ZLIB_INCLUDE_DIR ${ITKZLIB_INCLUDE_DIRS})
+set(ZLIB_INCLUDE_DIRS ${ITKZLIB_INCLUDE_DIRS})
 set(ZLIB_LIBRARY ${ITKZLIB_LIBRARIES})
+set(ZLIB_LIBRARIES ${ITKZLIB_LIBRARIES})
 
 # Configure GDCM privately so its options do not appear to the user.
 set(GDCM_LEGACY_REMOVE OFF CACHE INTERNAL "Remove all legacy code completely.")