ENH: Add minimum policy version specification for external FFTW builds

Set `CMAKE_POLICY_VERSION_MINIMUM` using `ITK_OLDEST_VALIDATED_POLICIES_VERSION`
in FFTW external builds.

FFTW 3.3.10 sets the cmake minimum version to 3.0 which is deprecated in
cmake versions greater than 4.0.

Explicitly setting the CMAKE_POLICY_VERSION_MINIMUM when configuring the build for fftw overrides
the test and avoids build failures due to too old of minimum versions.

Author: hjmjohnson

CMake/itkExternal_FFTW.cmake

@@ -91,7 +91,9 @@ if(NOT ITK_USE_SYSTEM_FFTW)
         -DCMAKE_C_FLAGS:STRING=${CMAKE_C_FLAGS}
         -DCMAKE_OSX_SYSROOT:PATH=${CMAKE_OSX_SYSROOT}
         -DCMAKE_OSX_DEPLOYMENT_TARGET:PATH=${CMAKE_OSX_DEPLOYMENT_TARGET}
-        -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=${CMAKE_POSITION_INDEPENDENT_CODE})
+        -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=${CMAKE_POSITION_INDEPENDENT_CODE}
+        -DCMAKE_POLICY_VERSION_MINIMUM:STRING=${ITK_OLDEST_VALIDATED_POLICIES_VERSION}
+    )
     # set(
     #   FFTW3f_DIR ${FFTW_STAGED_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/cmake/fftw3f
     #   CACHE PATH "Path to internally built single precision FFTW3Config.cmake"
@@ -145,7 +147,9 @@ if(NOT ITK_USE_SYSTEM_FFTW)
         -DCMAKE_OSX_SYSROOT:PATH=${CMAKE_OSX_SYSROOT}
         -DCMAKE_OSX_DEPLOYMENT_TARGET:PATH=${CMAKE_OSX_DEPLOYMENT_TARGET}
         -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=${CMAKE_POSITION_INDEPENDENT_CODE}
-      DEPENDS ${PROJ_FFTWD_DEPENDS} # Avoid potential collisions on install
+        -DCMAKE_POLICY_VERSION_MINIMUM:STRING=${ITK_OLDEST_VALIDATED_POLICIES_VERSION}
+      DEPENDS
+        ${PROJ_FFTWD_DEPENDS} # Avoid potential collisions on install
     )
     # set(
     #   FFTW3_DIR ${FFTW_STAGED_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/cmake/fftw3