To avoid noise during builds, unlike the GIT-CFLAGS rule which prints
"* new build flags or prefix" so the operator knows why all files are
being rebuilt when it changes, GIT-USER-AGENT generation is silent.
If this code breaks and a target depending on GIT-USER-AGENT ends up
being rebuilt when it shouldn't be, the full dependency chain can be
retrieved with "make --debug=b".
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>