Let's change this to a pattern rule. GNU's doc says suffix rules are obsolete:
http://www.gnu.org/software/make/manual/make.html#Suffix-Rules

Also, since we're not going to be generating .hpp$(PCH) files, they should be listed as .PHONY. And if we're going down that path, let's not confuse terminology by trying to call it $(PCH). That has a different meaning for one of our header files.