COMP: Pin DCMTK charset-conversion backend, disabling external iconv

DCMTK auto-detects an external libiconv and turns DCMTK_WITH_ICONV on when
one is found, which then runs an iconv try-compile during configure. In a
conda/pixi toolchain the conda iconv.h (which renames iconv -> libiconv) is
paired with the macOS SDK libiconv at link time, so the probe fails with
undefined _libiconv symbols and configure aborts.

ITK already selects the built-in oficonv backend
(DCMTK_ENABLE_BUILTIN_OFICONV_DATA), so the external-iconv probe is
redundant. Set DCMTK_WITH_ICONV=OFF explicitly in both the ICU and the
non-ICU branches so the backend is fixed by ITK rather than discovered from
whatever the build environment happens to provide.

Co-locate the three related options (DCMTK_ENABLE_BUILTIN_OFICONV_DATA,
DCMTK_WITH_ICU, DCMTK_WITH_ICONV) in CHARSET_CONVERSION_ARGS so their
interrelationship is visible in one place.

Author: hjmjohnson

Modules/ThirdParty/DCMTK/CMakeLists.txt

@@ -7,8 +7,22 @@ set(ITKDCMTK_NO_SRC 1)
 
 include(CMakeParseArguments)
 
+# oficonv is the fixed data backend; ICU and external iconv are mutually exclusive add-ons.
+set(
+  CHARSET_CONVERSION_ARGS
+  -DDCMTK_ENABLE_BUILTIN_OFICONV_DATA:BOOL=${DCMTK_ENABLE_BUILTIN_OFICONV_DATA}
+)
 if(DCMTK_USE_ICU)
   option(ITK_USE_SYSTEM_ICU "Use an installed version of ICU" OFF)
+  # ICU backend for both system and in-tree ICU; external iconv stays off.
+  list(
+    APPEND
+    CHARSET_CONVERSION_ARGS
+    -DDCMTK_WITH_STDLIBC_ICONV:BOOL=OFF
+    -DDCMTK_WITH_ICU:BOOL=ON
+    -DDCMTK_WITH_ICONV:BOOL=OFF
+    -DDCMTK_ENABLE_CHARSET_CONVERSION:STRING=ICU
+  )
   if(NOT ITK_USE_SYSTEM_ICU)
     set(ITKDCMTK_PREREQS ${ITKDCMTK_BINARY_DIR}/DCMTK_Prereqs)
     set(ITKDCMTK_ICU_LIBRARIES)
@@ -81,13 +95,7 @@ if(DCMTK_USE_ICU)
     else()
       set(ICU_ROOT_DIR ${ITKDCMTK_PREREQS})
     endif()
-    set(
-      CHARSET_CONVERSION_ARGS
-      -DDCMTK_WITH_STDLIBC_ICONV:BOOL=OFF
-      -DDCMTK_WITH_ICU:BOOL=ON
-      -DDCMTK_ENABLE_CHARSET_CONVERSION:STRING=ICU
-      -DICU_ROOT:PATH=${ICU_ROOT_DIR}
-    )
+    list(APPEND CHARSET_CONVERSION_ARGS -DICU_ROOT:PATH=${ICU_ROOT_DIR})
     itk_download_attempt_check(icu)
     ExternalProject_Add(
       icu
@@ -109,7 +117,13 @@ if(DCMTK_USE_ICU)
     set(ICU_DEPENDENCY icu)
   endif()
 else()
-  set(CHARSET_CONVERSION_ARGS -DDCMTK_WITH_ICU:BOOL=OFF)
+  # Built-in oficonv only; disable ICU and external iconv for a deterministic backend.
+  list(
+    APPEND
+    CHARSET_CONVERSION_ARGS
+    -DDCMTK_WITH_ICU:BOOL=OFF
+    -DDCMTK_WITH_ICONV:BOOL=OFF
+  )
 endif()
 
 set(
@@ -480,7 +494,6 @@ endforeach()
       -DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_INSTALL_PREFIX}
       -DCMAKE_INSTALL_LIBDIR:PATH=${CMAKE_INSTALL_LIBDIR}
       -DCMAKE_INSTALL_BINDIR:PATH=${CMAKE_INSTALL_BINDIR}
-      -DDCMTK_ENABLE_BUILTIN_OFICONV_DATA:BOOL=${DCMTK_ENABLE_BUILTIN_OFICONV_DATA}
       ${CHARSET_CONVERSION_ARGS}
     DEPENDS
       ${JPEG_DEPENDENCY}