config/configure: add and use bup_find_prog; remove unused MF_PATH_INCLUDEs.
Pull all the common program search code into bup_find_prog() and use
it everywhere. This also makes configure's output a bit more
consistent in style/content.
Make it obvious that at the moment bup doesn't pay any attention to a
PYTHON or GIT environment variable value. i.e. bup currently ignores a
setting like PYTHON=python2.7.
Bup also doesn't use MF_PATH_INCLUDE settings yet, so remove them.
Signed-off-by: Rob Browning <rlb@defaultvalue.org>