Change path to CMakeLists.txt to be relative to root, not pwd (#4420)

* Change path to CMakeLists.txt to be relative to root, not pwd

* Changes requested during code review

* Revert back to old naming of root_cmakelists_dir
* Make relative directory more clear in docs

* Revert change causing build_type AttributeError

* Fix forgotten abs_path var

* Update CLHEP with new relative path

* Update more packages with new root_cmakelists_dir syntax

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

@@ -31,10 +31,6 @@ class Clfft(CMakePackage):
     homepage = "https://github.com/clMathLibraries/clFFT"
     url      = "https://github.com/clMathLibraries/clFFT/archive/v2.12.2.tar.gz"
 
-    @property
-    def root_cmakelists_dir(self):
-        return join_path(self.stage.source_path, 'src')
-
     version('2.12.2', '9104d85f9f2f3c58dd8efc0e4b06496f')
 
     variant('client', default=True,
@@ -43,6 +39,8 @@ def root_cmakelists_dir(self):
     depends_on('opencl@1.2:')
     depends_on('boost@1.33.0:', when='+client')
 
+    root_cmakelists_dir = 'src'
+
     def cmake_args(self):
         spec = self.spec