Makefile: Track changes to LDFLAGS and relink when necessary

Makefile: Track changes to LDFLAGS and relink when necessary

Some profiling tools (e.g., google-perftools and mutrace) work by
linking in a new library into the executables. When using these tools
it is convenient to only relink instead of doing a full make clean;
make cycle.

This change complements the auto-detection of changes to CFLAGS that
we already have. Tracking of more variables that affect the build can
be added when the need arise.

Signed-off-by: Fredrik Kuivinen <frekui@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>