build-sys: create parent directory for $(PATHFILES)

When building outside the source tree there is nothing to guarantee the
target directory exists before writing to it. Most of the time this just
happens to work because something else creates the directory but not
always. For example:

    $ mkdir build
    $ cd build
    $ ../configure --disable-dependency-tracking
    $ make libuuid/uuid.pc
      GEN      libuuid/uuid.pc
    /bin/sh: line 2: libuuid/uuid.pc.tmp: No such file or directory
    make: *** [libuuid/uuid.pc] Error 1