Make gawk a dependency of dune

The duneproject script uses awk. On my machine it fails because system awk
is messed up by the spack set-up LD_LIBRARY_PATH.

packages/dune/package.py

@@ -171,6 +171,7 @@ class Dune(CMakePackage):
 
     extends('python')
     #option
+    depends_on('gawk')
     depends_on('cmake@3.1:', type='build')
     depends_on('mpi')
     depends_on('blas',   when='+blas')