PNetCDF package: enable option required by NetCDF (#11420)

The --enable-relax-coord-bound option was added in 1.8.0. NetCDF
4.6.3 enabled relax coords by default and required it from PNetCDF.
PNetCDF 1.10.0 enabled it by default but for versions between 1.8.0
and 1.10.0 it should be enabled explicitly.

var/spack/repos/builtin/packages/parallel-netcdf/package.py

@@ -70,6 +70,9 @@ def configure_args(self):
         if '~fortran' in spec:
             args.append('--disable-fortran')
 
+        if spec.satisfies('@1.8.0:'):
+            args.append('--enable-relax-coord-bound')
+
         return args
 
     def install(self, spec, prefix):