GeoModelKernel: Compile GeoAlignableTransform.cxx with optimization, even in debug builds.

GeoAlignableTransform::getTransform makes heavy use of Eigen, and is very slow in debug builds. Tweak the build so that this is compiled with optimization, with full inlining, even in debug builds.

This speeds up the RecJobTransforms unit test by 15% (from 900s to 750s).

GeoModelCore/GeoModelKernel/src/GeoAlignableTransform.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 #include "GeoModelKernel/GeoAlignableTransform.h"
@@ -19,6 +19,14 @@ GeoAlignableTransform::~GeoAlignableTransform()
   delete m_delta;
 }
 
+#if defined(FLATTEN) && defined(__GNUC__)
+// We compile this package with optimization, even in debug builds; otherwise,
+// the heavy use of Eigen makes it too slow.  However, from here we may call
+// to out-of-line Eigen code that is linked from other DSOs; in that case,
+// it would not be optimized.  Avoid this by forcing all Eigen code
+// to be inlined here if possible.
+__attribute__ ((flatten))
+#endif
 GeoTrf::Transform3D GeoAlignableTransform::getTransform(const GeoVAlignmentStore* store) const
 {
   const GeoTrf::Transform3D* delta = (store==nullptr ? m_delta : store->getDelta(this));