STYLE: Simplify AVX detection guards to #ifndef form

Replace the redundant `!defined(__AVX__) || !__AVX__` pattern with the
conventional `#ifndef __AVX__` form.  All major compilers (GCC, Clang,
MSVC /arch:AVX) define __AVX__ as 1 (never 0) when AVX is active, so
the `|| !__AVX__` branch is dead code.

Addresses Greptile P2 review comment.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hjmjohnson

CMake/itkExternal_FFTW.cmake

@@ -170,15 +170,15 @@ if(NOT ITK_USE_SYSTEM_FFTW)
       # pass -DFFTW_ENABLE_AVX2=ON explicitly.
       unset(_fftw_compiler_targets_avx CACHE)
       check_c_source_compiles(
-        "#if !defined(__AVX__) || !__AVX__\n#error AVX not enabled\n#endif\nint main(void){return 0;}"
+        "#ifndef __AVX__\n#error AVX not enabled\n#endif\nint main(void){return 0;}"
         _fftw_compiler_targets_avx
       )
       if(_fftw_compiler_targets_avx)
         set(_fftw_default_avx ON)
       endif()
       unset(_fftw_compiler_targets_avx2 CACHE)
       check_c_source_compiles(
-        "#if !defined(__AVX2__) || !__AVX2__\n#error AVX2 not enabled\n#endif\nint main(void){return 0;}"
+        "#ifndef __AVX2__\n#error AVX2 not enabled\n#endif\nint main(void){return 0;}"
         _fftw_compiler_targets_avx2
       )
       if(_fftw_compiler_targets_avx2)