Fix broken make install behaviour

CMake uses rpath to make it easier to run built executables from the build
tree without installing prerequisite libraries first. This means that when
installing, CMake will re-link the binary in order to change the rpath so
it works properly in an install. Unfortunately, it's not smart enough to know
to re-embed the systemlib section, so the installed `hhvm` binary does not
have a systemlib section and doesn't work.

This commit adds a cmake function `HHVM_INSTALL` that effectively bypasses the
standard install command to ensure that the binary is not relinked. To help
with rpaths, this also checks for the existance of the `chrpath` tool. If it
exists, it uses it to either remove or replace the rpath entry in the file,
otherwise it just leaves the rpath entry alone.

This allows `make install` to work again.