COMP: Suppress GCC -Wmaybe-uninitialized false positive on Eigen 5

The warning is purely from the Eigen 5.0.1 update changing instantiation/
inlining. Eigen's own CMakeLists.txt already adds -Wno-maybe-uninitialized
for its own builds (line 444, with a comment that GCC 12+ emits false
positives), and DisableStupidWarnings.h doesn't cover this one. The fix is
to suppress at the ITK consumer site, mirroring Eigen's stance.

GCC 13.3 inlines the analytical 3x3 fast path of Eigen 5's
SelfAdjointEigenSolver into SymmetricEigenAnalysisFixedDimension::
ComputeEigenValues<3, Matrix<float,3,3>, FixedArray<float,3>,
Matrix<float,3,3>>, called transitively from
SymmetricSecondRankTensor<float,3>::ComputeEigenValues. GCC's IPA cannot
prove the third element of solver.m_eivalues is fully written through
Eigen's deeply-templated dispatch and emits [-Wmaybe-uninitialized] on
itkSymmetricEigenAnalysis.h:1033 (auto eigenValues = solver.eigenvalues();).

Add ITK_GCC_SUPPRESS_Wmaybe_uninitialized macro to itkMacro.h alongside
the existing ITK_GCC_SUPPRESS_W* family, and apply ITK_GCC_PRAGMA_PUSH /
ITK_GCC_PRAGMA_POP around the affected lines, with // clang-format off/on
to preserve the macro layout (clang-format otherwise folds the SUPPRESS
macro into the adjacent declaration line and breaks parsing).

Verified locally on Apple Silicon arm64 inside a
ghcr.io/catthehacker/ubuntu:full-24.04 container (gcc 13.3.0, the same
toolchain as the failing CI runner): full ITK build (2539 ninja edges)
goes from 2 -Wmaybe-uninitialized warnings on this line to 0.

Author: hjmjohnson

Modules/Core/Common/include/itkMacro.h

@@ -109,12 +109,14 @@ namespace itk
 #  define ITK_GCC_SUPPRESS_Wfloat_equal ITK_PRAGMA(GCC diagnostic ignored "-Wfloat-equal")
 #  define ITK_GCC_SUPPRESS_Wformat_nonliteral ITK_PRAGMA(GCC diagnostic ignored "-Wformat-nonliteral")
 #  define ITK_GCC_SUPPRESS_Warray_bounds ITK_PRAGMA(GCC diagnostic ignored "-Warray-bounds")
+#  define ITK_GCC_SUPPRESS_Wmaybe_uninitialized ITK_PRAGMA(GCC diagnostic ignored "-Wmaybe-uninitialized")
 #else
 #  define ITK_GCC_PRAGMA_PUSH
 #  define ITK_GCC_PRAGMA_POP
 #  define ITK_GCC_SUPPRESS_Wfloat_equal
 #  define ITK_GCC_SUPPRESS_Wformat_nonliteral
 #  define ITK_GCC_SUPPRESS_Warray_bounds
+#  define ITK_GCC_SUPPRESS_Wmaybe_uninitialized
 #endif
 
 // For Clang only (and not GCC):

Modules/Core/Common/include/itkSymmetricEigenAnalysis.h

@@ -1029,8 +1029,16 @@ class ITK_TEMPLATE_EXPORT SymmetricEigenAnalysisFixedDimension
       }
     }
     using EigenSolverType = Eigen::SelfAdjointEigenSolver<EigenLibMatrixType>;
+    // GCC's IPA cannot prove that Eigen 5's analytical 3x3 SelfAdjointEigenSolver
+    // fully writes m_eivalues; Eigen's own CMakeLists adds -Wno-maybe-uninitialized
+    // for the same reason (see Modules/ThirdParty/Eigen3/.../CMakeLists.txt).
+    // clang-format off
+    ITK_GCC_PRAGMA_PUSH
+    ITK_GCC_SUPPRESS_Wmaybe_uninitialized
     const EigenSolverType solver(inputMatrix, Eigen::EigenvaluesOnly);
     auto                  eigenValues = solver.eigenvalues();
+    ITK_GCC_PRAGMA_POP
+    // clang-format on
     if (m_OrderEigenValues == EigenValueOrderEnum::OrderByMagnitude)
     {
       detail::sortEigenValuesByMagnitude(eigenValues, VDimension);