debian/GNUmakefile - fix strange build warnings
debian/GNUmakefile - fix strange build warnings

When making debian packages, the build was generating this concerning
messages:

/bin/tar: rm -f pcp.postrm: Cannot stat: No such file or directory
/bin/tar: cat pcp.postrm.head configversions pcp.postrm.tail > pcp.postrm: Cannot stat: No such file or directory
/bin/tar: Exiting with failure status due to previous errors

from the build/tar directory (we do descend there, even for debian
package builds).

Turns out pcp.postrm was not being made early enough in the build
which led to the build/deb/pcp-3.8.10/build/pcp.src control file
being polluted with make noise ...

rm -f pcp.postrm
cat pcp.postrm.head configversions pcp.postrm.tail > pcp.postrm

which confused the tar build when it expected the pcp.src file to
contain the names of source files.

wuliming/pcp,edwardt/pcp,adfernandes/pcp,tjanez/pcp,mbaldessari/pcp,andyvand/cygpcpfans,edwardt/pcp,tjanez/pcp,wuliming/pcp,aeg-aeg/pcpfans,edwardt/pcp,wuliming/pcp,aeg-aeg/pcpfans,andyvand/cygpcpfans,adfernandes/pcp,edwardt/pcp,prasincs/pcp,prasincs/pcp,aeg-aeg/pcpfans,prasincs/pcp,adfernandes/pcp,wuliming/pcp,andyvand/cygpcpfans,andyvand/cygpcpfans,edwardt/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,tjanez/pcp,prasincs/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,edwardt/pcp,tjanez/pcp,aeg-aeg/pcpfans,tjanez/pcp,adfernandes/pcp,mbaldessari/pcp,mbaldessari/pcp,wuliming/pcp,tjanez/pcp,wuliming/pcp,andyvand/cygpcpfans,prasincs/pcp,andyvand/cygpcpfans,mbaldessari/pcp,aeg-aeg/pcpfans,mbaldessari/pcp,andyvand/cygpcpfans,tjanez/pcp,adfernandes/pcp,edwardt/pcp,mbaldessari/pcp,edwardt/pcp,prasincs/pcp,tjanez/pcp,prasincs/pcp,adfernandes/pcp,andyvand/cygpcpfans,adfernandes/pcp,prasincs/pcp,aeg-aeg/pcpfans