auto-detect changed prefix and/or changed build flags

auto-detect changed prefix and/or changed build flags

Detect changed prefix and/or changed build flags in the middle
of the build (or between 'make' and 'make install'), and if change
is detected, make sure all objects are compiled with same build
flags and same prefix, thus avoiding inconsistent/broken build.

[jc: removed otherwise unnecessary Makefile target to test the
 change this patch introduces. ]

Signed-off-by: Yakov Lerner <redacted>
Signed-off-by: Junio C Hamano <redacted>