Add f18 package (#9183)

* Add f18 package

* Fix flake8 errors

* Install the f18 binary into prefix.bin by hand

* Add spack-build into the path

* Fix the license header

var/spack/repos/builtin/packages/f18/package.py

@@ -0,0 +1,23 @@
+# Copyright 2013-2018 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class F18(CMakePackage):
+    """F18 is a front-end for Fortran intended to replace the existing front-end
+    in the Flang compiler"""
+
+    homepage = "https://github.com/flang-compiler/f18"
+
+    git      = "https://github.com/flang-compiler/f18"
+
+    version('develop', branch='master')
+
+    depends_on('llvm@6.0.0+clang', when='@develop')
+
+    def install(self, spec, prefix):
+        mkdirp(prefix.bin)
+        install("spack-build/tools/f18/f18", prefix.bin)