COMP: Enable position-independent code for src/ executables

Modern GCC on Ubuntu 22.04+ defaults to building PIE (position-
independent executables). When the ITK shared libraries are compiled
with -fPIC but the executables are not built with -fPIE, the typeinfo
for itk::ExceptionObject cannot be resolved at link time:

  undefined reference to typeinfo for itk::ExceptionObject
  warning: creating DT_TEXTREL in a PIE

Setting CMAKE_POSITION_INDEPENDENT_CODE=ON in src/CMakeLists.txt
ensures compatible code generation between the executables and the
ITK shared libraries they link against.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hjmjohnson

src/CMakeLists.txt

@@ -1,6 +1,11 @@
 include_directories("${VariationalRegistration_SOURCE_DIR}/include")
 include_directories("${VariationalRegistration_SOURCE_DIR}/src")
 
+# Modern GCC on Ubuntu 22.04+ defaults to PIE executables. Ensure
+# position-independent code so typeinfo lookups for ITK exceptions
+# resolve correctly against the -fPIC shared libraries.
+set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+
 # Test if ITK is built with FFTW. FFTWD is recommended and required for the
 # tests with elastic or curvature regularisation to pass.
 if(ITK_USE_FFTWF AND NOT ITK_USE_FFTWD)