Cleanup of Makefile to be able to set Flags from packaging

Based on [@gregoryp work](http://anonscm.debian.org/gitweb/?p=collab-maint/beanstalkd.git;a=blob;f=debian/patches/0002-Sanitize-Makefile.patch;h=185bb088cf0f72b5122341b7219cd27c10fb25d5;hb=HEAD)
This let use set a different PRFEFIX and CFLAGS from debhelper script (to build debian package).
This will not impact other usage.