setup.py: More Unique build_base (#355)

This likely can race in superbuilds of dependent projects.

setup.py

@@ -24,7 +24,7 @@ def initialize_options(self):
         # clashes with directories many developers have in their source trees;
         # this can create confusing results with "pip install .", which clones
         # the whole source tree by default
-        self.build_base = "_tmppythonbuild"
+        self.build_base = os.path.join("_tmppythonbuild", "amrex")
 
     def run(self):
         # remove existing build directory