[BUILD] Added support for 'make install'

To be flexible while installing haproxy following variables have been
added to the Makefile:
- DESTDIR useful i.e. while installing in a sandbox (not set by default)
- PREFIX  defines the default install prefix (default: /usr/local)
- SBINDIR defines the dir the haproxy binary gets installed
  (default: $PREFIX/sbin)
1 file changed