GMT: add conflict for GCC 11 (#26684)

var/spack/repos/builtin/packages/gmt/package.py

@@ -62,6 +62,10 @@ class Gmt(Package):
 
     depends_on('graphicsmagick', type='test')
 
+    # https://github.com/spack/spack/issues/26661
+    conflicts('%gcc@11:', when='@:5',
+              msg='GMT 5 cannot be built with GCC 11+, try a newer GMT or older GCC')
+
     # https://github.com/GenericMappingTools/gmt/pull/3603
     patch('regexp.patch', when='@6.1.0')
     patch('type.patch', when='@4.5.9')