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

@@ -7,6 +7,14 @@ set(ITKDCMTK_NO_SRC 1)
 
 include(CMakeParseArguments)
 
+# Character-set-conversion options for the DCMTK build, kept together so the
+# oficonv / ICU / external-iconv relationship is visible in one place. The
+# built-in oficonv module is always the data backend; ICU and external libiconv
+# are mutually exclusive add-ons selected below.
+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)
   if(NOT ITK_USE_SYSTEM_ICU)
@@ -81,10 +89,12 @@ if(DCMTK_USE_ICU)
     else()
       set(ICU_ROOT_DIR ${ITKDCMTK_PREREQS})
     endif()
-    set(
+    list(
+      APPEND
       CHARSET_CONVERSION_ARGS
       -DDCMTK_WITH_STDLIBC_ICONV:BOOL=OFF
       -DDCMTK_WITH_ICU:BOOL=ON
+      -DDCMTK_WITH_ICONV:BOOL=OFF
       -DDCMTK_ENABLE_CHARSET_CONVERSION:STRING=ICU
       -DICU_ROOT:PATH=${ICU_ROOT_DIR}
     )
@@ -109,7 +119,14 @@ 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 the external libiconv probe so the
+  # backend is fixed regardless of what the environment happens to provide.
+  list(
+    APPEND
+    CHARSET_CONVERSION_ARGS
+    -DDCMTK_WITH_ICU:BOOL=OFF
+    -DDCMTK_WITH_ICONV:BOOL=OFF
+  )
 endif()
 
 set(
@@ -480,7 +497,6 @@ 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}