ENH: Specify CMAKE_OSX_ARCHITECTURES for external FFTW builds

Add `CMAKE_OSX_ARCHITECTURES` to the configuration to ensure proper
architecture selection during FFTW external builds.

This is needed to get the correct architecture builds when rosetta x86_64 is
used for the desired architectures.

Author: hjmjohnson

CMake/itkExternal_FFTW.cmake

@@ -91,6 +91,7 @@ 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=${CMAKE_OSX_DEPLOYMENT_TARGET}
+        -DCMAKE_OSX_ARCHITECTURES:STRING=${CMAKE_OSX_ARCHITECTURES}
         -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=${CMAKE_POSITION_INDEPENDENT_CODE}
         -DCMAKE_POLICY_VERSION_MINIMUM:STRING=${ITK_OLDEST_VALIDATED_POLICIES_VERSION}
     )
@@ -146,6 +147,7 @@ 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=${CMAKE_OSX_DEPLOYMENT_TARGET}
+        -DCMAKE_OSX_ARCHITECTURES:STRING=${CMAKE_OSX_ARCHITECTURES}
         -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=${CMAKE_POSITION_INDEPENDENT_CODE}
         -DCMAKE_POLICY_VERSION_MINIMUM:STRING=${ITK_OLDEST_VALIDATED_POLICIES_VERSION}
       DEPENDS