[Backport PR-3722] COMP: Ensure client project can find externally build ITK module

This commit fixes a regression introduced in 5b09d54 (COMP: Allow
install lib directory name changes).

Client project not explicitly setting CMAKE_INSTALL_LIBDIR (or including
the "GNUInstallDirs" CMake module) would fail to find externally build
ITK module.

This commit addresses this by setting CMAKE_INSTALL_LIBDIR default value
to "lib". This approach is consistent with how the CMAKE_INSTALL_LIBDIR
default value is set in the top-level CMakeLists.txt.

Before this change, there was discrepancy between the value of
"/path/to/<module>.cmake" used when configuring the file and the
one used when looking up the file following a call
to "find_package(ITK COMPONENTS <module>)".

For example:

  Location of the configured file:
  /path/to/ITK-build//cmake/ITK-5.3/Modules/Ultrasound.cmake

vs

  Location expected by "itk_module_load()"
  /path/to/ITK-build/lib/cmake/ITK-5.3/Modules/Ultrasound.cmake

Author: jcfr

CMake/ITKModuleExternal.cmake

@@ -50,6 +50,12 @@ if(NOT ITK_BINARY_DIR)
   set(ITK_BINARY_DIR ${ITK_DIR})
 endif()
 
+# Set default value for project that are not using the
+# "GNUInstallDirs" CMake module.
+if(NOT DEFINED CMAKE_INSTALL_LIBDIR)
+  set(CMAKE_INSTALL_LIBDIR "lib")
+endif()
+
 # The default path when not wrapping.  Restore standard build location
 # if python wrapping is turned on, and then turned off.
 if(NOT CMAKE_LIBRARY_OUTPUT_DIRECTORY)