COMP: Enable -Wshadow warnings for GCC and clang

Suggested by Bradley Lowekamp at SimpleITK/SimpleITK#2104 (comment)

Aims to reproduce those warnings from SimpleITK/monarch-i-06c02d82ab38530ea-Linux at https://open.cdash.org/viewBuildError.php?type=1&buildid=9568906

Author: N-Dekker

CMakeLists.txt

@@ -81,6 +81,10 @@ include(elastixExportTarget)
 
 if (MSVC)
   add_compile_options(/W3)
+else()
+  add_compile_options(
+    -Wshadow
+    )
 endif()
 
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